diff --git a/dictation_server/src/features/accounts/accounts.service.spec.ts b/dictation_server/src/features/accounts/accounts.service.spec.ts
index 7335dee..baefc94 100644
--- a/dictation_server/src/features/accounts/accounts.service.spec.ts
+++ b/dictation_server/src/features/accounts/accounts.service.spec.ts
@@ -3920,7 +3920,7 @@ describe('deleteWorktype', () => {
     source = null;
   });
 
-  it('WorktypeIDを削除できる', async () => {
+  it('WorktypeIDを削除できること', 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 });
@@ -3966,6 +3966,51 @@ describe('deleteWorktype', () => {
     }
   });
 
+  it('指定されたWorktypeIDがアカウントのActiveWorktypeIDの場合、削除できること', 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 { id: worktypeId1 } = await createWorktype(
+      source,
+      account.id,
+      'worktype1',
+      'description1',
+      true,
+    );
+    await createOptionItems(source, worktypeId1);
+
+    // 作成したデータを確認
+    {
+      const worktypes = await getWorktypes(source, account.id);
+      const optionItems = await getOptionItems(source);
+      const accounts = await getAccounts(source);
+
+      expect(worktypes.length).toBe(1);
+      expect(worktypes[0].id).toBe(worktypeId1);
+      expect(worktypes[0].custom_worktype_id).toBe('worktype1');
+      expect(optionItems.length).toBe(10);
+      expect(accounts.length).toBe(1);
+      expect(accounts[0].active_worktype_id).toBe(worktypeId1);
+    }
+
+    await service.deleteWorktype(context, admin.external_id, worktypeId1);
+
+    //実行結果を確認
+    {
+      const worktypes = await getWorktypes(source, account.id);
+      const optionItems = await getOptionItems(source);
+      const accounts = await getAccounts(source);
+      expect(worktypes.length).toBe(0);
+      expect(optionItems.length).toBe(0);
+      expect(accounts.length).toBe(1);
+      expect(accounts[0].active_worktype_id).toBe(null);
+    }
+  });
+
   it('指定したWorktypeIDが登録されていない場合、400エラーとなること', 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/worktypes/worktypes.repository.service.ts b/dictation_server/src/repositories/worktypes/worktypes.repository.service.ts
index 7fef32e..88f3bd3 100644
--- a/dictation_server/src/repositories/worktypes/worktypes.repository.service.ts
+++ b/dictation_server/src/repositories/worktypes/worktypes.repository.service.ts
@@ -176,6 +176,19 @@ export class WorktypesRepositoryService {
         throw new WorktypeIdNotFoundError(`Worktype is not found. id: ${id}`);
       }
 
+      // アカウントのActiveWorktypeIDが削除対象のワークタイプIDの場合はActiveWorktypeIDをnullに更新
+      const accountRepo = entityManager.getRepository(Account);
+      const account = await accountRepo.findOne({
+        where: { id: accountId },
+      });
+
+      if (account?.active_worktype_id === id) {
+        await accountRepo.update(
+          { id: accountId },
+          { active_worktype_id: null },
+        );
+      }
+
       // ワークタイプがワークフローに紐づいている場合はエラー
       const workflowRepo = entityManager.getRepository(Workflow);
       const workflows = await workflowRepo.find({