diff --git a/dictation_client/src/pages/WorkTypeIdSettingPage/index.tsx b/dictation_client/src/pages/WorkTypeIdSettingPage/index.tsx
index 4fa599c..b2b8715 100644
--- a/dictation_client/src/pages/WorkTypeIdSettingPage/index.tsx
+++ b/dictation_client/src/pages/WorkTypeIdSettingPage/index.tsx
@@ -140,7 +140,7 @@ const WorktypeIdSettingPage: React.FC = (): JSX.Element => {
                     <select
                       name="Active Worktype"
                       className={styles.formInput}
-                      value={activeWorktypeId}
+                      value={activeWorktypeId ?? ""}
                       onChange={onChangeActiveWorktype}
                     >
                       {/* eslint-disable-next-line jsx-a11y/control-has-associated-label */}
diff --git a/dictation_server/src/features/accounts/accounts.service.spec.ts b/dictation_server/src/features/accounts/accounts.service.spec.ts
index 1ac8181..6f61cf0 100644
--- a/dictation_server/src/features/accounts/accounts.service.spec.ts
+++ b/dictation_server/src/features/accounts/accounts.service.spec.ts
@@ -4402,6 +4402,37 @@ describe('updateActiveWorktype', () => {
     }
   });
 
+  it('アカウントのActiveWorktypeIDをNULLに更新できる（WorkTypeID⇒NULL）', async () => {
+    const module = await makeTestingModule(source);
+    // 第五階層のアカウント作成
+    const { account, admin } = await makeTestAccount(source, { tier: 5 });
+
+    const service = module.get<AccountsService>(AccountsService);
+    const context = makeContext(admin.external_id);
+
+    const worktype1 = await createWorktype(
+      source,
+      account.id,
+      'worktype1',
+      'description1',
+      true,
+    );
+
+    //作成したデータを確認
+    {
+      const beforeAccount = await getAccount(source, account.id);
+      expect(beforeAccount.active_worktype_id).toBe(worktype1.id);
+    }
+
+    await service.updateActiveWorktype(context, admin.external_id, undefined);
+
+    //実行結果を確認
+    {
+      const { active_worktype_id } = await getAccount(source, account.id);
+      expect(active_worktype_id).toBe(null);
+    }
+  });
+
   it('自アカウント内に指定されたIDのWorktypeIDが存在しない場合、400エラーとなること（WorkTypeIDが存在しない場合）', async () => {
     const module = await makeTestingModule(source);
     // 第五階層のアカウント作成
diff --git a/dictation_server/src/repositories/accounts/accounts.repository.service.ts b/dictation_server/src/repositories/accounts/accounts.repository.service.ts
index 8ce21f2..6636f21 100644
--- a/dictation_server/src/repositories/accounts/accounts.repository.service.ts
+++ b/dictation_server/src/repositories/accounts/accounts.repository.service.ts
@@ -771,26 +771,34 @@ export class AccountsRepositoryService {
   /**
    * ActiveWorktypeIdを更新する
    * @param accountId
-   * @param id ActiveWorktypeIdの内部ID
+   * @param [id] ActiveWorktypeIdの内部ID
    * @returns active worktype id
    */
-  async updateActiveWorktypeId(accountId: number, id: number): Promise<void> {
+  async updateActiveWorktypeId(
+    accountId: number,
+    id?: number | undefined,
+  ): Promise<void> {
     return await this.dataSource.transaction(async (entityManager) => {
       const worktypeRepo = entityManager.getRepository(Worktype);
       const accountRepo = entityManager.getRepository(Account);
 
-      // 自アカウント内に指定IDのワークタイプが存在するか確認
-      const worktype = await worktypeRepo.findOne({
-        where: { account_id: accountId, id: id },
-      });
+      if (id) {
+        // 自アカウント内に指定IDのワークタイプが存在するか確認
+        const worktype = await worktypeRepo.findOne({
+          where: { account_id: accountId, id: id },
+        });
 
-      // ワークタイプが存在しない場合はエラー
-      if (!worktype) {
-        throw new WorktypeIdNotFoundError('Worktype is not found. id: ${id}');
+        // ワークタイプが存在しない場合はエラー
+        if (!worktype) {
+          throw new WorktypeIdNotFoundError('Worktype is not found. id: ${id}');
+        }
       }
 
       // アカウントのActiveWorktypeIDを更新
-      await accountRepo.update({ id: accountId }, { active_worktype_id: id });
+      await accountRepo.update(
+        { id: accountId },
+        { active_worktype_id: id ?? null },
+      );
     });
   }
 }