diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7051a35b78..378598bd8d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,7 +7,7 @@
 -
 
 ### Server
--
+- Fix: 既にミュートしているスレッドに対して再度スレッドミュートを作成しようとするとサーバーエラーになる問題を修正
 
 
 ## 2026.7.0
diff --git a/packages/backend/src/server/api/endpoints/notes/thread-muting/create.ts b/packages/backend/src/server/api/endpoints/notes/thread-muting/create.ts
index 7c0dddb827..143e823367 100644
--- a/packages/backend/src/server/api/endpoints/notes/thread-muting/create.ts
+++ b/packages/backend/src/server/api/endpoints/notes/thread-muting/create.ts
@@ -30,6 +30,12 @@ export const meta = {
 			code: 'NO_SUCH_NOTE',
 			id: '5ff67ada-ed3b-2e71-8e87-a1a421e177d2',
 		},
+
+		alreadyMuting: {
+			message: 'You are already muting that thread.',
+			code: 'ALREADY_MUTING',
+			id: 'c146e22d-1141-4b31-b28d-176371014d18',
+		},
 	},
 } as const;
 
@@ -59,14 +65,18 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				throw err;
 			});
 
-			const _mutedNotes = await this.notesRepository.find({
-				where: [{
-					id: note.threadId ?? note.id,
-				}, {
+			// Check if already muting
+			const exist = await this.noteThreadMutingsRepository.exists({
+				where: {
 					threadId: note.threadId ?? note.id,
-				}],
+					userId: me.id,
+				},
 			});
 
+			if (exist) {
+				throw new ApiError(meta.errors.alreadyMuting);
+			}
+
 			await this.noteThreadMutingsRepository.insert({
 				id: this.idService.gen(),
 				threadId: note.threadId ?? note.id,