diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6e32249ee7..c08cb036f7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -34,6 +34,7 @@
 - Fix: 投稿通知がオンでもダイレクト投稿はユーザーに通知されないようにされました
 - Fix: ユーザタイムラインの「ノート」選択時にリノートが混ざり込んでしまうことがある問題の修正 #12306
 - Fix: ActivityPubに関するセキュリティの向上
+- Fix: 非公開の投稿に対して返信できないように
 
 ## 2023.11.0
 
diff --git a/packages/backend/src/server/api/endpoints/notes/create.ts b/packages/backend/src/server/api/endpoints/notes/create.ts
index df02d3acb7..513b77b98a 100644
--- a/packages/backend/src/server/api/endpoints/notes/create.ts
+++ b/packages/backend/src/server/api/endpoints/notes/create.ts
@@ -70,6 +70,12 @@ export const meta = {
 			id: '749ee0f6-d3da-459a-bf02-282e2da4292c',
 		},
 
+		cannotReplyToInvisibleNote: {
+			message: 'You cannot reply to an invisible Note.',
+			code: 'CANNOT_REPLY_TO_AN_INVISIBLE_NOTE',
+			id: 'b98980fa-3780-406c-a935-b6d0eeee10d1',
+		},
+
 		cannotReplyToPureRenote: {
 			message: 'You can not reply to a pure Renote.',
 			code: 'CANNOT_REPLY_TO_A_PURE_RENOTE',
@@ -276,6 +282,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					throw new ApiError(meta.errors.noSuchReplyTarget);
 				} else if (isPureRenote(reply)) {
 					throw new ApiError(meta.errors.cannotReplyToPureRenote);
+				} else if (!await this.noteEntityService.isVisibleForMe(reply, me.id)) {
+					throw new ApiError(meta.errors.cannotReplyToInvisibleNote);
 				}
 
 				// Check blocking
diff --git a/packages/backend/test/e2e/timelines.ts b/packages/backend/test/e2e/timelines.ts
index 760bb8a574..73c446444b 100644
--- a/packages/backend/test/e2e/timelines.ts
+++ b/packages/backend/test/e2e/timelines.ts
@@ -152,6 +152,7 @@ describe('Timelines', () => {
 
 			await api('/following/create', { userId: bob.id }, alice);
 			await api('/following/create', { userId: carol.id }, alice);
+			await api('/following/create', { userId: carol.id }, bob);
 			await api('/following/update', { userId: bob.id, withReplies: true }, alice);
 			await sleep(1000);
 			const carolNote = await post(carol, { text: 'hi', visibility: 'followers' });