diff --git a/packages/backend/src/core/activitypub/ApInboxService.ts b/packages/backend/src/core/activitypub/ApInboxService.ts
index d0d206760cb9..d91eed34988a 100644
--- a/packages/backend/src/core/activitypub/ApInboxService.ts
+++ b/packages/backend/src/core/activitypub/ApInboxService.ts
@@ -29,7 +29,7 @@ import { bindThis } from '@/decorators.js';
 import type { MiRemoteUser } from '@/models/User.js';
 import { isNotNull } from '@/misc/is-not-null.js';
 import { GlobalEventService } from '@/core/GlobalEventService.js';
-import { getApHrefNullable, getApId, getApIds, getApType, isAccept, isActor, isAdd, isAnnounce, isBlock, isCollection, isCollectionOrOrderedCollection, isCreate, isDelete, isFlag, isFollow, isLike, isMove, isPost, isReject, isRemove, isTombstone, isUndo, isUpdate, validActor, validPost } from './type.js';
+import { getApHrefNullable, getApId, getApIds, getApType, isAccept, isActor, isAdd, isAnnounce, isBlock, isCollection, isCollectionOrOrderedCollection, isCreate, isDelete, isFlag, isFollow, isLike, isMove, isPost, isReject, isRemove, isTombstone, isUndo, isUpdate, isValidActor, isValidPost } from './type.js';
 import { ApNoteService } from './models/ApNoteService.js';
 import { ApLoggerService } from './ApLoggerService.js';
 import { ApDbResolverService } from './ApDbResolverService.js';
@@ -471,9 +471,9 @@ export class ApInboxService {
 			formerType = 'Note';
 		}
 
-		if (validPost.includes(formerType)) {
+		if (isValidPost(formerType)) {
 			return await this.deleteNote(actor, uri);
-		} else if (validActor.includes(formerType)) {
+		} else if (isValidActor(formerType)) {
 			return await this.deleteActor(actor, uri);
 		} else {
 			return `Unknown type ${formerType}`;
diff --git a/packages/backend/src/core/activitypub/models/ApNoteService.ts b/packages/backend/src/core/activitypub/models/ApNoteService.ts
index a2e47327e27b..24bfd63d1776 100644
--- a/packages/backend/src/core/activitypub/models/ApNoteService.ts
+++ b/packages/backend/src/core/activitypub/models/ApNoteService.ts
@@ -25,7 +25,7 @@ import { bindThis } from '@/decorators.js';
 import { checkHttps } from '@/misc/check-https.js';
 import { IdentifiableError } from '@/misc/identifiable-error.js';
 import { isNotNull } from '@/misc/is-not-null.js';
-import { getOneApId, getApId, getOneApHrefNullable, validPost, isEmoji, getApType } from '../type.js';
+import { getOneApId, getApId, getOneApHrefNullable, isEmoji, getApType, isPost } from '../type.js';
 import { ApLoggerService } from '../ApLoggerService.js';
 import { ApMfmService } from '../ApMfmService.js';
 import { ApDbResolverService } from '../ApDbResolverService.js';
@@ -80,7 +80,7 @@ export class ApNoteService {
 	public validateNote(object: IObject, uri: string): Error | null {
 		const expectHost = this.utilityService.extractDbHost(uri);
 
-		if (!validPost.includes(getApType(object))) {
+		if (!isPost(object)) {
 			return new IdentifiableError('d450b8a9-48e4-4dab-ae36-f4db763fda7c', `invalid Note: invalid object type ${getApType(object)}`);
 		}
 
diff --git a/packages/backend/src/core/activitypub/type.ts b/packages/backend/src/core/activitypub/type.ts
index 5b6c6c8ca6cb..94f80ee799a3 100644
--- a/packages/backend/src/core/activitypub/type.ts
+++ b/packages/backend/src/core/activitypub/type.ts
@@ -61,10 +61,10 @@ export function getApId(value: string | IObject): string {
 /**
  * Get ActivityStreams Object type
  */
-export function getApType(value: IObject): string {
+export function getApType(value: IObject): string | null {
 	if (typeof value.type === 'string') return value.type;
 	if (Array.isArray(value.type) && typeof value.type[0] === 'string') return value.type[0];
-	throw new Error('cannot detect type');
+	return null;
 }
 
 export function getOneApHrefNullable(value: ApObject | undefined): string | undefined {
@@ -106,13 +106,17 @@ export interface IOrderedCollection extends IObject {
 	orderedItems: ApObject;
 }
 
-export const validPost = ['Note', 'Question', 'Article', 'Audio', 'Document', 'Image', 'Page', 'Video', 'Event'];
+// つまるところこれをしないとnull許容でincludesできないので必須
+type ValidArrayIncludesNullable = readonly (string | null)[];
+const validPost = ['Note', 'Question', 'Article', 'Audio', 'Document', 'Image', 'Page', 'Video', 'Event'] as const;
+export const isValidPost = (type: string | null): boolean =>
+	(validPost as ValidArrayIncludesNullable).includes(type);
 
 export const isPost = (object: IObject): object is IPost =>
-	validPost.includes(getApType(object));
+	isValidPost(getApType(object));
 
 export interface IPost extends IObject {
-	type: 'Note' | 'Question' | 'Article' | 'Audio' | 'Document' | 'Image' | 'Page' | 'Video' | 'Event';
+	type: (typeof validPost)[number];
 	source?: {
 		content: string;
 		mediaType: string;
@@ -154,13 +158,15 @@ export interface ITombstone extends IObject {
 export const isTombstone = (object: IObject): object is ITombstone =>
 	getApType(object) === 'Tombstone';
 
-export const validActor = ['Person', 'Service', 'Group', 'Organization', 'Application'];
+const validActor = ['Person', 'Service', 'Group', 'Organization', 'Application'] as const;
+export const isValidActor = (type: string | null): boolean =>
+	(validActor as ValidArrayIncludesNullable).includes(type);
 
 export const isActor = (object: IObject): object is IActor =>
-	validActor.includes(getApType(object));
+	isValidActor(getApType(object));
 
 export interface IActor extends IObject {
-	type: 'Person' | 'Service' | 'Organization' | 'Group' | 'Application';
+	type: (typeof validActor)[number];
 	name?: string;
 	preferredUsername?: string;
 	manuallyApprovesFollowers?: boolean;
@@ -240,12 +246,16 @@ export interface IKey extends IObject {
 	publicKeyPem: string | Buffer;
 }
 
-export interface IApDocument extends IObject {
-	type: 'Audio' | 'Document' | 'Image' | 'Page' | 'Video';
-}
+const validDocument = ['Audio', 'Document', 'Image', 'Page', 'Video'] as const;
+export const isValidDocument = (type: string | null): boolean =>
+	(validDocument as ValidArrayIncludesNullable).includes(type);
 
 export const isDocument = (object: IObject): object is IApDocument =>
-	['Audio', 'Document', 'Image', 'Page', 'Video'].includes(getApType(object));
+	isValidDocument(getApType(object));
+
+export interface IApDocument extends IObject {
+	type: (typeof validDocument)[number];
+}
 
 export interface IApImage extends IApDocument {
 	type: 'Image';