diff --git a/packages/backend/src/core/AnnouncementService.ts b/packages/backend/src/core/AnnouncementService.ts
index 95899c2ccc..37c0a8d52b 100644
--- a/packages/backend/src/core/AnnouncementService.ts
+++ b/packages/backend/src/core/AnnouncementService.ts
@@ -14,6 +14,7 @@ import { IdService } from '@/core/IdService.js';
 import { AnnouncementEntityService } from '@/core/entities/AnnouncementEntityService.js';
 import { GlobalEventService } from '@/core/GlobalEventService.js';
 import { ModerationLogService } from '@/core/ModerationLogService.js';
+import { IdentifiableError } from '@/misc/identifiable-error.js';
 
 @Injectable()
 export class AnnouncementService {
@@ -67,6 +68,16 @@ export class AnnouncementService {
 
 	@bindThis
 	public async create(values: Partial<MiAnnouncement>, moderator?: MiUser): Promise<{ raw: MiAnnouncement; packed: Packed<'Announcement'> }> {
+		if (values.display === 'dialog') {
+			// Check how many active dialog queries already exist, to enforce a limit
+			const query = this.announcementsRepository.createQueryBuilder('announcement');
+			query.andWhere('announcement.isActive = true');
+			const count = await query.getCount();
+			if (count >= 5) {
+				throw new IdentifiableError('c0d15f15-f18e-4a40-bcb1-f310d58204ee', 'Too many dialog announcements.');
+			}
+		}
+
 		const announcement = await this.announcementsRepository.insertOne({
 			id: this.idService.gen(),
 			updatedAt: null,
diff --git a/packages/backend/src/server/api/endpoints/admin/announcements/create.ts b/packages/backend/src/server/api/endpoints/admin/announcements/create.ts
index 8da39810e9..3052fdf2fc 100644
--- a/packages/backend/src/server/api/endpoints/admin/announcements/create.ts
+++ b/packages/backend/src/server/api/endpoints/admin/announcements/create.ts
@@ -4,8 +4,10 @@
  */
 
 import { Injectable } from '@nestjs/common';
+import { ApiError } from '@/server/api/error.js';
 import { Endpoint } from '@/server/api/endpoint-base.js';
 import { AnnouncementService } from '@/core/AnnouncementService.js';
+import { IdentifiableError } from '@/misc/identifiable-error.js';
 
 export const meta = {
 	tags: ['admin'],
@@ -48,6 +50,14 @@ export const meta = {
 			},
 		},
 	},
+
+	errors: {
+		tooManyDialogs: {
+			message: 'Cannot create the announcement because there are too many active dialog-style announcements.',
+			code: 'DIALOG_LIMIT_EXCEEDED',
+			id: '7c1bc084-9c14-4bcf-a910-978cd8e99b5a',
+		},
+	},
 } as const;
 
 export const paramDef = {
@@ -73,22 +83,28 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		private announcementService: AnnouncementService,
 	) {
 		super(meta, paramDef, async (ps, me) => {
-			const { raw, packed } = await this.announcementService.create({
-				updatedAt: null,
-				title: ps.title,
-				text: ps.text,
-				/* eslint-disable-next-line @typescript-eslint/prefer-nullish-coalescing -- 空の文字列の場合、nullを渡すようにするため */
-				imageUrl: ps.imageUrl || null,
-				icon: ps.icon,
-				display: ps.display,
-				forExistingUsers: ps.forExistingUsers,
-				silence: ps.silence,
-				needConfirmationToRead: ps.needConfirmationToRead,
-				confetti: ps.confetti,
-				userId: ps.userId,
-			}, me);
-
-			return packed;
+			try {
+				const { raw, packed } = await this.announcementService.create({
+					updatedAt: null,
+					title: ps.title,
+					text: ps.text,
+					/* eslint-disable-next-line @typescript-eslint/prefer-nullish-coalescing -- 空の文字列の場合、nullを渡すようにするため */
+					imageUrl: ps.imageUrl || null,
+					icon: ps.icon,
+					display: ps.display,
+					forExistingUsers: ps.forExistingUsers,
+					silence: ps.silence,
+					needConfirmationToRead: ps.needConfirmationToRead,
+					confetti: ps.confetti,
+					userId: ps.userId,
+				}, me);
+				return packed;
+			} catch (e) {
+				if (e instanceof IdentifiableError) {
+					if (e.id === 'c0d15f15-f18e-4a40-bcb1-f310d58204ee') throw new ApiError(meta.errors.tooManyDialogs);
+				}
+				throw e;
+			}
 		});
 	}
 }