diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index e243be1..2b600b8 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -2,8 +2,11 @@ import React from "react";
 import { Link, NavLink, Route, Routes, useLocation } from "react-router-dom";
 import Timer from "./components/Timer";
 import ShowsPage from "./pages/ShowsPage";
+import TimeSyncNotice from "./components/TimeSyncNotice";
 import "./index.css";
 
+const TIME_SYNC_OFF_THRESHOLD = 1000;
+
 export default function App() {
   const [open, setOpen] = React.useState(false);
   const loc = useLocation();
@@ -22,6 +25,7 @@ export default function App() {
     <div className={`site ${open ? "sidebar-open" : ""}`}>
       {/* Top-left header (outside the card) */}
       <header className="site-header">
+
         <button className="burger" aria-label="メニュー" onClick={() => setOpen(v => !v)}>
           <svg className="burger-icon" viewBox="0 0 24 24" width="22" height="22" aria-hidden="true">
             <path d="M4 7h16M4 12h16M4 17h16"
@@ -31,6 +35,8 @@ export default function App() {
         </button>
         <Link to="/" className="brand">Watch Party</Link>
       </header>
+      {/* Time sync banner (checks every 5 min; shows if |skew| > 500ms) */}
+      <TimeSyncNotice thresholdMs={TIME_SYNC_OFF_THRESHOLD} lang="ja" />
 
       {/* Sidebar (full-height) */}
       <aside className={`sidebar ${open ? "open" : ""}`} aria-hidden={!open}>
diff --git a/frontend/src/components/TimeSyncNotice.tsx b/frontend/src/components/TimeSyncNotice.tsx
new file mode 100644
index 0000000..67a75e3
--- /dev/null
+++ b/frontend/src/components/TimeSyncNotice.tsx
@@ -0,0 +1,62 @@
+import { useEffect, useState } from "react";
+import { useTimeSkew } from "../hooks/useTimeSkew";
+
+function formatMs(ms: number) {
+    const sign = ms > 0 ? "+" : "";
+    return `${sign}${ms}ms`;
+}
+
+export default function TimeSyncNotice({
+    thresholdMs = 500,
+    endpoint,
+    intervalMs,
+    lang = "ja",
+}: {
+    thresholdMs?: number;
+    endpoint?: string;
+    intervalMs?: number;
+    lang?: "ja" | "en";
+}) {
+    // removed `error`
+    const { skewMs, rttMs, recheck } = useTimeSkew({ endpoint, intervalMs });
+
+    const [dismissed, setDismissed] = useState<boolean>(() => {
+        try { return sessionStorage.getItem("timesync.dismissed") === "1"; } catch { return false; }
+    });
+
+    useEffect(() => {
+        if (skewMs != null && Math.abs(skewMs) <= thresholdMs && dismissed) {
+            setDismissed(false);
+            try { sessionStorage.removeItem("timesync.dismissed"); } catch { }
+        }
+    }, [skewMs, thresholdMs, dismissed]);
+
+    if (dismissed || skewMs == null || Math.abs(skewMs) <= thresholdMs) return null;
+
+    const ahead = skewMs > 0;
+    const msgJa = ahead
+        ? `端末の時計が正確な時刻より ${formatMs(skewMs)} 進んでいます（往復遅延 ${rttMs ?? "-"}ms）`
+        : `端末の時計が正確な時刻より ${formatMs(-skewMs)} 遅れています（往復遅延 ${rttMs ?? "-"}ms）`;
+    const msgEn = ahead
+        ? `Your device clock is ${formatMs(skewMs)} ahead of the correct time (RTT ${rttMs ?? "-"}ms).`
+        : `Your device clock is ${formatMs(-skewMs)} behind the correct time (RTT ${rttMs ?? "-"}ms).`;
+
+    const onClose = () => {
+        setDismissed(true);
+        try { sessionStorage.setItem("timesync.dismissed", "1"); } catch { }
+    };
+
+    return (
+        <div className="timesync-banner" role="status" aria-live="polite">
+            <div className="timesync-msg">{lang === "ja" ? msgJa : msgEn}</div>
+            <div className="timesync-actions">
+                <button className="timesync-btn" onClick={() => recheck?.()}>
+                    {lang === "ja" ? "再測定" : "Re-check"}
+                </button>
+                <button className="timesync-close" onClick={onClose} aria-label={lang === "ja" ? "閉じる" : "Close"}>
+                    ×
+                </button>
+            </div>
+        </div>
+    );
+}
\ No newline at end of file
diff --git a/frontend/src/components/Timer.tsx b/frontend/src/components/Timer.tsx
index de79772..54224e0 100644
--- a/frontend/src/components/Timer.tsx
+++ b/frontend/src/components/Timer.tsx
@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useRef, useState } from "react";
+import { useServerClock } from "../hooks/useServerClock";
 
 // ===== Config & fallbacks =====
 const TIMEZONE = "Asia/Tokyo"; // JST (UTC+09)
@@ -6,6 +7,7 @@ const API_URL = "/api/v1/current";
 const FALLBACK_START_HOUR = 19;
 const FALLBACK_START_MINUTE = 25;
 const FALLBACK_END_SECONDS = 300;
+const TIME_SYNC_INTERVAL = 60_000;
 // ==============================
 
 type ApiSchedule = {
@@ -71,44 +73,6 @@ function jstToUtcMs(y: number, m: number, d: number, hh: number, mm: number, ss
     return Date.UTC(y, m - 1, d, hh - 9, mm, ss, 0);
 }
 
-/** Compact "current time in JST" readout used under the digits */
-function NowJst() {
-    const [time, setTime] = useState(() =>
-        new Intl.DateTimeFormat("en-GB", {
-            timeZone: TIMEZONE,
-            hour: "2-digit",
-            minute: "2-digit",
-            second: "2-digit",
-            hour12: false,
-        }).format(new Date())
-    );
-
-    useEffect(() => {
-        const fmt = new Intl.DateTimeFormat("en-GB", {
-            timeZone: TIMEZONE,
-            hour: "2-digit",
-            minute: "2-digit",
-            second: "2-digit",
-            hour12: false,
-        });
-        let t: number | null = null;
-        const tick = () => {
-            setTime(fmt.format(new Date()));
-            const msToNext = 1000 - (Date.now() % 1000);
-            t = window.setTimeout(tick, msToNext + 5);
-        };
-        tick();
-        return () => { if (t) clearTimeout(t); };
-    }, []);
-
-    return (
-        <div className="timer-now">
-            <span className="muted">現在時刻</span>
-            <span className="now-digits">{time}</span>
-        </div>
-    );
-}
-
 async function loadSchedule(signal?: AbortSignal) {
     const res = await fetch(API_URL, { cache: "no-store", signal });
     if (!res.ok) throw new Error(`HTTP ${res.status}`);
@@ -132,6 +96,10 @@ export default function Timer() {
     const [phase, setPhase] = useState<"waiting" | "running" | "ended">("waiting");
     const [untilStart, setUntilStart] = useState(0);
     const timerRef = useRef<number | null>(null);
+    const { nowMs, ready, error: timeError } = useServerClock({
+        endpoint: `${import.meta.env.BASE_URL}api/time`,  // works under /watch-party/ in prod, /api in dev
+        refreshMs: TIME_SYNC_INTERVAL,
+    });
 
     // fetch schedule
     useEffect(() => {
@@ -161,27 +129,25 @@ export default function Timer() {
 
     // tick per second
     useEffect(() => {
+        if (!ready) return;
         const tick = () => {
-            const nowMs = Date.now();
-
-            if (nowMs < startUtcMs) {
+            const now = nowMs();
+            if (now < startUtcMs) {
                 setPhase("waiting");
-                setUntilStart(Math.ceil((startUtcMs - nowMs) / 1000));
+                setUntilStart(Math.ceil((startUtcMs - now) / 1000));
                 setElapsed(0);
             } else {
-                const secs = Math.floor((nowMs - startUtcMs) / 1000);
+                const secs = Math.floor((now - startUtcMs) / 1000);
                 const clamped = Math.min(secs, endSeconds);
                 setElapsed(clamped);
                 setPhase(clamped >= endSeconds ? "ended" : "running");
             }
-
-            const msToNext = 1000 - (nowMs % 1000);
+            const msToNext = 1000 - (now % 1000);
             timerRef.current = window.setTimeout(tick, msToNext + 5);
         };
-
         tick();
         return () => { if (timerRef.current) window.clearTimeout(timerRef.current); };
-    }, [startUtcMs, endSeconds]);
+    }, [startUtcMs, endSeconds, nowMs]);
 
     // status labels
     const startLabel = useMemo(() => {
@@ -190,8 +156,9 @@ export default function Timer() {
         });
         return fmt.format(new Date(startUtcMs));
     }, [startUtcMs]);
-
     const progress = endSeconds > 0 ? Math.min(100, Math.round((elapsed / endSeconds) * 100)) : 0;
+    const showPrestartCountdown = phase === "waiting" && untilStart > 0 && untilStart <= 10;
+    const hasHours = endSeconds >= 3600 || elapsed >= 3600;
 
     // nicety: update tab title while running
     useEffect(() => {
@@ -245,8 +212,6 @@ export default function Timer() {
         // eslint-disable-next-line react-hooks/exhaustive-deps
     }, [phase]); // runs when phase flips to "ended"
 
-    const showPrestartCountdown = phase === "waiting" && untilStart > 0 && untilStart <= 10;
-    const hasHours = endSeconds >= 3600 || elapsed >= 3600;
 
     return (
         <div className="timer-block">
@@ -265,8 +230,10 @@ export default function Timer() {
                     {phase === "waiting" && `Starts ${startLabel} JST`}
                     {phase === "ended" && "Ended"}
                 </span>
-                <NowJst />
+                <NowJst getNowMs={nowMs} />
             </div>
+            {/* show time endpoint error */}
+            {timeError && <div className="timer-status" style={{ color: "#f88" }}>時刻取得エラー: {timeError}</div>}
 
             {/* Hero digits */}
             <div className={`timer-hero ${hasHours ? "has-hours" : ""}`} role="timer" aria-live="off">
@@ -309,4 +276,31 @@ export default function Timer() {
             )}
         </div>
     );
+}
+
+/** Compact "current time in JST" readout used under the digits */
+function NowJst({ getNowMs }: { getNowMs: () => number }) {
+    const [time, setTime] = useState("");
+
+    useEffect(() => {
+        const fmt = new Intl.DateTimeFormat("en-GB", {
+            timeZone: TIMEZONE, hour: "2-digit", minute: "2-digit", second: "2-digit", hour12: false,
+        });
+        let t: number | null = null;
+        const tick = () => {
+            const now = new Date(getNowMs());
+            setTime(fmt.format(now));
+            const msToNext = 1000 - (getNowMs() % 1000);
+            t = window.setTimeout(tick, msToNext + 5);
+        };
+        tick();
+        return () => { if (t) clearTimeout(t); };
+    }, [getNowMs]);
+
+    return (
+        <div className="timer-now">
+            <span className="muted">現在時刻</span>
+            <span className="now-digits">{time}</span>
+        </div>
+    );
 }
\ No newline at end of file
diff --git a/frontend/src/hooks/useServerClock.ts b/frontend/src/hooks/useServerClock.ts
new file mode 100644
index 0000000..076327a
--- /dev/null
+++ b/frontend/src/hooks/useServerClock.ts
@@ -0,0 +1,52 @@
+import { useCallback, useEffect, useRef, useState } from "react";
+
+/** Uses /api/time => { now: <ms UTC> } and returns a server-correct "nowMs()" */
+export function useServerClock(opts?: {
+    endpoint?: string;
+    refreshMs?: number;
+    enabled?: boolean;
+}) {
+    const {
+        endpoint = "/api/time",
+        refreshMs = 60_000,
+        enabled = true,
+    } = opts || {};
+
+    const [offsetMs, setOffsetMs] = useState<number | null>(null); // client - server
+    const [error, setError] = useState<string | null>(null);
+    const timerRef = useRef<number | null>(null);
+
+    const syncOnce = useCallback(async () => {
+        try {
+            setError(null);
+            const t0 = Date.now();
+            const res = await fetch(endpoint, { cache: "no-store" });
+            const t1 = Date.now();
+            if (!res.ok) throw new Error(`HTTP ${res.status}`);
+            const data = await res.json();
+            if (typeof data?.now !== "number") throw new Error("Bad time payload");
+            const s = data.now;
+            const offset = Math.round(((t0 + t1) / 2) - s); // (+) client ahead, (-) client behind
+            setOffsetMs(offset);
+        } catch (e: any) {
+            setError(e?.message || "time sync failed");
+        }
+    }, [endpoint]);
+
+    useEffect(() => {
+        if (!enabled) return;
+        syncOnce();
+        if (refreshMs > 0) {
+            timerRef.current = window.setInterval(syncOnce, refreshMs);
+        }
+        return () => { if (timerRef.current) clearInterval(timerRef.current); };
+    }, [enabled, refreshMs, syncOnce]);
+
+    const nowMs = useCallback(() => {
+        if (offsetMs == null) return Date.now();         // first paint fallback
+        return Date.now() - offsetMs;                    // server-correct time
+    }, [offsetMs]);
+
+    const ready = offsetMs != null;
+    return { nowMs, ready, error, resync: syncOnce, offsetMs };
+}
\ No newline at end of file
diff --git a/frontend/src/hooks/useTimeSkew.ts b/frontend/src/hooks/useTimeSkew.ts
new file mode 100644
index 0000000..60ca0e9
--- /dev/null
+++ b/frontend/src/hooks/useTimeSkew.ts
@@ -0,0 +1,87 @@
+import { useEffect, useRef, useState } from "react";
+
+/**
+ * Measures client clock skew vs server time {now: <ms since epoch, UTC>}.
+ * NTP-style estimate:
+ *   t0 = client ms before request
+ *   s  = server ms from JSON
+ *   t1 = client ms after response
+ *   offset ≈ ((t0 + t1)/2) - s
+ * Positive offset => client is AHEAD by that many ms.
+ */
+export function useTimeSkew(opts?: {
+    endpoint?: string;     // e.g., "/api/time" or `${import.meta.env.BASE_URL}api/time`
+    intervalMs?: number;   // how often to recheck; default 5 min
+    samples?: number;      // take N measurements and median; default 1
+    enabled?: boolean;     // allow turning off; default true
+}) {
+    const {
+        endpoint = "/api/time",
+        intervalMs = 5 * 60_000,
+        samples = 1,
+        enabled = true,
+    } = opts || {};
+
+    const [skewMs, setSkewMs] = useState<number | null>(null);
+    const [rttMs, setRttMs] = useState<number | null>(null);
+    const [error, setError] = useState<string | null>(null);
+    const timerRef = useRef<number | null>(null);
+
+    const measureOnce = async () => {
+        const t0 = Date.now();
+        const res = await fetch(endpoint, { cache: "no-store" });
+        const t1 = Date.now();
+        if (!res.ok) throw new Error(`HTTP ${res.status}`);
+        const data = await res.json();
+
+        if (typeof data?.now !== "number") {
+            throw new Error("Bad time payload (expecting { now: number })");
+        }
+        const serverMs = data.now;
+        const rtt = t1 - t0;
+        const offset = Math.round(((t0 + t1) / 2) - serverMs);
+        return { offset, rtt };
+    };
+
+    const measure = async () => {
+        try {
+            setError(null);
+            if (samples <= 1) {
+                const { offset, rtt } = await measureOnce();
+                setSkewMs(offset);
+                setRttMs(rtt);
+                return;
+            }
+            // multiple samples → median offset, min RTT (more stable)
+            const offsets: number[] = [];
+            const rtts: number[] = [];
+            for (let i = 0; i < samples; i++) {
+                const { offset, rtt } = await measureOnce();
+                offsets.push(offset);
+                rtts.push(rtt);
+                if (i < samples - 1) await new Promise(r => setTimeout(r, 120)); // tiny gap
+            }
+            offsets.sort((a, b) => a - b);
+            const median = offsets[Math.floor(offsets.length / 2)];
+            const minRtt = Math.min(...rtts);
+            setSkewMs(median);
+            setRttMs(minRtt);
+        } catch (e: any) {
+            setError(e?.message || "Time sync failed");
+        }
+    };
+
+    useEffect(() => {
+        if (!enabled) return;
+        measure(); // initial check
+        if (intervalMs > 0) {
+            timerRef.current = window.setInterval(measure, intervalMs);
+        }
+        return () => {
+            if (timerRef.current) window.clearInterval(timerRef.current);
+        };
+        // eslint-disable-next-line react-hooks/exhaustive-deps
+    }, [endpoint, intervalMs, samples, enabled]);
+
+    return { skewMs, rttMs, error, recheck: measure };
+}
\ No newline at end of file
diff --git a/frontend/src/index.css b/frontend/src/index.css
index fba08e6..cc8d6e3 100644
--- a/frontend/src/index.css
+++ b/frontend/src/index.css
@@ -30,6 +30,7 @@ html, body, #root {
 }
 /* Button container */
 .burger{
+  grid-area: burger;
   display:grid;                 /* centers SVG */
   place-items:center;
   width:40px; height:40px;      /* good touch target */
@@ -51,7 +52,7 @@ html, body, #root {
 .burger:hover{ background:rgba(255,255,255,.12); }
 .burger:focus-visible{ outline:2px solid var(--accent); outline-offset:2px; }
 
-.brand { color: var(--text); text-decoration:none; font-weight:800; }
+.brand { color: var(--text); text-decoration:none; font-weight:800; grid-area: title;  justify-self: start; }
 .drawer {
   position:absolute; left:16px; top:56px; display:flex; flex-direction:column;
   gap:8px; padding:12px; border-radius:12px; background:rgba(15,21,32,0.98);
@@ -169,14 +170,21 @@ kbd {
   /* push-mode: the content will slide right when sidebar is open */
 }
 
-.site-header {
+.site-header{
   position: fixed;
   z-index: 50;
   top: 0; left: 16px; right: 16px;
-  display: flex; gap: 12px; align-items: center;
   height: var(--header-h);
+
+  /* three areas: banner (fills), burger (left), title (left of center) */
+  display: grid;
+  grid-template-columns: auto auto 1fr;        /* burger | title | banner */
+  grid-template-areas: "burger title banner";
+  align-items: center;
+  column-gap: 12px;
 }
 
+
 .brand {
   font-weight: 900;
   color: var(--text);
@@ -384,6 +392,57 @@ kbd {
 
 .footer a { text-decoration: underline; }
 
+.site-header { position: fixed; z-index: 60; top: 0; left: 16px; right: 16px; height: var(--header-h); display:flex; align-items:center; gap:12px; }
+
+/* Centered badge that sizes to its content */
+.timesync-banner{
+  position: fixed;
+  z-index: 70;
+  top: calc(env(safe-area-inset-top, 0px) + var(--header-h) + 6px);
+  left: 50%;
+  transform: translateX(-50%);
+
+  display: inline-flex;
+  align-items: center;
+  gap: 10px;
+  padding: 8px 12px;
+
+  width: max-content;              /* shrink to content */
+  max-width: 92vw;                 /* keep inside viewport */
+  white-space: nowrap;             /* single line (desktop) */
+
+  border-radius: 12px;
+  background: rgba(255,200,0,.12);
+  border: 1px solid rgba(255,200,0,.35);
+  color: #ffe9a6;
+  box-sizing: border-box;
+}
+
+/* message shouldn’t force stretch */
+.timesync-msg { flex: 0 1 auto; }
+
+/* buttons stay tidy */
+.timesync-actions{
+  margin-left: 8px;
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+}
+.timesync-btn,
+.timesync-close{
+  appearance: none;
+  border: 1px solid rgba(255,255,255,.18);
+  background: rgba(255,255,255,.08);
+  color: inherit;
+  font: inherit;
+  line-height: 1;
+  height: 30px;
+  padding: 6px 10px;
+  border-radius: 10px;
+  cursor: pointer;
+}
+.timesync-close{ width: 30px; padding: 0; }
+
 @keyframes pop {
   from { transform: scale(0.98); opacity: 0.0; }
   to   { transform: scale(1);    opacity: 1.0; }
@@ -391,6 +450,38 @@ kbd {
 @media (prefers-reduced-motion: reduce) {
   .progress-fill { transition: none; }
   .countdown-wrap {animation: none;}
+  .timesync-banner {
+    flex-wrap: wrap;
+    gap: 8px 10px;
+  }
+  .timesync-actions {
+    width: 100%;
+    justify-content: flex-end;
+  }
+}
+@media (max-width: 720px){
+  .site-header{
+    grid-template-columns: auto 1fr;
+    grid-template-areas:
+      "burger title"
+      "banner banner";
+    row-gap: 8px;
+    height: calc(var(--header-h) + 40px);
+  }
+}
+@media (max-width: 640px){
+  .timesync-banner{
+    display: flex;
+    flex-wrap: wrap;
+    row-gap: 6px;
+    white-space: normal;               /* allow wrap */
+    max-width: min(900px, 92vw);
+  }
+  .timesync-msg{ flex: 1 1 100%; }     /* text on first row */
+  .timesync-actions{ margin-left: auto; }
+  .site-content{
+    padding-top: calc(var(--header-h) + 72px); /* push card down so it won't sit under the banner */
+  }
 }
 @container (max-width: 420px) {
   .timer-hero { font-size: clamp(22px, 11cqw, 72px); letter-spacing: 0.25px; }