diff --git a/src/remote/activitypub/act/create/index.ts b/src/remote/activitypub/act/create/index.ts
index d210aa4c5..7ab4c2aba 100644
--- a/src/remote/activitypub/act/create/index.ts
+++ b/src/remote/activitypub/act/create/index.ts
@@ -16,8 +16,6 @@ export default async (actor: IRemoteUser, activity): Promise<void> => {
 
 	log(`Create: ${uri}`);
 
-	// TODO: 同じURIをもつものが既に登録されていないかチェック
-
 	const resolver = new Resolver();
 
 	let object;
diff --git a/src/remote/activitypub/act/create/note.ts b/src/remote/activitypub/act/create/note.ts
index 8b22f1b47..253478b6f 100644
--- a/src/remote/activitypub/act/create/note.ts
+++ b/src/remote/activitypub/act/create/note.ts
@@ -4,23 +4,31 @@ import * as debug from 'debug';
 import Resolver from '../../resolver';
 import Post, { IPost } from '../../../../models/post';
 import createPost from '../../../../services/post/create';
-import { IRemoteUser, isRemoteUser } from '../../../../models/user';
+import { IRemoteUser } from '../../../../models/user';
 import resolvePerson from '../../resolve-person';
 import createImage from './image';
+import config from '../../../../config';
 
 const log = debug('misskey:activitypub');
 
+/**
+ * 投稿作成アクティビティを捌きます
+ */
 export default async function createNote(resolver: Resolver, actor: IRemoteUser, note, silent = false): Promise<IPost> {
-	if (
-		('attributedTo' in note && actor.account.uri !== note.attributedTo) ||
-		typeof note.id !== 'string'
-	) {
+	if (typeof note.id !== 'string') {
 		log(`invalid note: ${JSON.stringify(note, null, 2)}`);
 		throw new Error('invalid note');
 	}
 
+	// 既に同じURIを持つものが登録されていないかチェックし、登録されていたらそれを返す
+	const exist = await Post.findOne({ uri: note.id });
+	if (exist) {
+		return exist;
+	}
+
 	log(`Creating the Note: ${note.id}`);
 
+	//#region 添付メディア
 	const media = [];
 	if ('attachment' in note && note.attachment != null) {
 		// TODO: attachmentは必ずしもImageではない
@@ -30,21 +38,31 @@ export default async function createNote(resolver: Resolver, actor: IRemoteUser,
 			media.push(created);
 		});
 	}
+	//#endregion
 
+	//#region リプライ
 	let reply = null;
 	if ('inReplyTo' in note && note.inReplyTo != null) {
-		const inReplyToPost = await Post.findOne({ uri: note.inReplyTo.id || note.inReplyTo });
+		// リプライ先の投稿がMisskeyに登録されているか調べる
+		const uri: string = note.inReplyTo.id || note.inReplyTo;
+		const inReplyToPost = uri.startsWith(config.url + '/')
+			? await Post.findOne({ _id: uri.split('/').pop() })
+			: await Post.findOne({ uri });
+
 		if (inReplyToPost) {
 			reply = inReplyToPost;
 		} else {
+			// 無かったらフェッチ
 			const inReplyTo = await resolver.resolve(note.inReplyTo) as any;
-			const actor = await resolvePerson(inReplyTo.attributedTo);
-			if (isRemoteUser(actor)) {
-				// TODO: silentを常にtrueにしてはならない
-				reply = await createNote(resolver, actor, inReplyTo);
-			}
+
+			// リプライ先の投稿の投稿者をフェッチ
+			const actor = await resolvePerson(inReplyTo.attributedTo) as IRemoteUser;
+
+			// TODO: silentを常にtrueにしてはならない
+			reply = await createNote(resolver, actor, inReplyTo);
 		}
 	}
+	//#endregion
 
 	const { window } = new JSDOM(note.content);
 
diff --git a/src/remote/activitypub/renderer/note.ts b/src/remote/activitypub/renderer/note.ts
index e45b10215..b971a5395 100644
--- a/src/remote/activitypub/renderer/note.ts
+++ b/src/remote/activitypub/renderer/note.ts
@@ -19,11 +19,11 @@ export default async (user: IUser, post: IPost) => {
 
 		if (inReplyToPost !== null) {
 			const inReplyToUser = await User.findOne({
-				_id: post.userId,
+				_id: inReplyToPost.userId,
 			});
 
 			if (inReplyToUser !== null) {
-				inReplyTo = `${config.url}@${inReplyToUser.username}/${inReplyToPost._id}`;
+				inReplyTo = inReplyToPost.uri || `${config.url}/@${inReplyToUser.username}/${inReplyToPost._id}`;
 			}
 		}
 	} else {