diff --git a/src/prelude/array.ts b/src/prelude/array.ts
index 54f7081712..42f05dc0c9 100644
--- a/src/prelude/array.ts
+++ b/src/prelude/array.ts
@@ -18,6 +18,10 @@ export function erase<T>(x: T, xs: T[]): T[] {
 	return xs.filter(y => x !== y);
 }
 
+export function setDifference<T>(xs: T[], ys: T[]): T[] {
+	return xs.filter(x => !ys.includes(x));
+}
+
 export function unique<T>(xs: T[]): T[] {
 	return [...new Set(xs)];
 }
diff --git a/src/remote/activitypub/models/note.ts b/src/remote/activitypub/models/note.ts
index f9380eb4f4..ce9a4daf16 100644
--- a/src/remote/activitypub/models/note.ts
+++ b/src/remote/activitypub/models/note.ts
@@ -13,7 +13,7 @@ import htmlToMFM from '../../../mfm/html-to-mfm';
 import Emoji from '../../../models/emoji';
 import { ITag } from './tag';
 import { toUnicode } from 'punycode';
-import { unique } from '../../../prelude/array';
+import { unique, concat, setDifference } from '../../../prelude/array';
 
 const log = debug('misskey:activitypub');
 
@@ -179,15 +179,9 @@ async function extractEmojis(tags: ITag[], host_: string) {
 	);
 }
 
-async function extractMentionedUsers(actor: IRemoteUser, to: string[], cc: string[], resolver: Resolver ) {
-	let uris = [] as string[];
-
-	if (to) uris.concat(to);
-	if (cc) uris.concat(cc);
-
-	uris = uris.filter(x => x !== 'https://www.w3.org/ns/activitystreams#Public');
-	uris = uris.filter(x => x !== `${actor.uri}/followers`);
-	uris = unique(uris);
+async function extractMentionedUsers(actor: IRemoteUser, to: string[], cc: string[], resolver: Resolver) {
+	const ignoreUris = ['https://www.w3.org/ns/activitystreams#Public', `${actor.uri}/followers`];
+	const uris = setDifference(unique(concat([to || [], cc || []])), ignoreUris);
 
 	const users = await Promise.all(
 		uris.map(async uri => await resolvePerson(uri, null, resolver).catch(() => null))