diff --git a/CHANGELOG.md b/CHANGELOG.md
index aabd9f000c..b885a25bab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
 ### General
 - Feat: コンテンツの表示にログインを必須にできるように
 - Feat: 過去のノートを非公開化/フォロワーのみ表示可能にできるように
+- Fix: アナウンスメントを作成ときにWebUIフォームの画像URLを後悔できないのを修正 (/admin/announcement/create)
 - Enhance: 依存関係の更新
 - Enhance: l10nの更新
 
diff --git a/packages/backend/src/core/AnnouncementService.ts b/packages/backend/src/core/AnnouncementService.ts
index d4fcf19439..a9f6731977 100644
--- a/packages/backend/src/core/AnnouncementService.ts
+++ b/packages/backend/src/core/AnnouncementService.ts
@@ -72,7 +72,7 @@ export class AnnouncementService {
 			updatedAt: null,
 			title: values.title,
 			text: values.text,
-			imageUrl: values.imageUrl,
+			imageUrl: values.imageUrl || null,
 			icon: values.icon,
 			display: values.display,
 			forExistingUsers: values.forExistingUsers,
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 2dae1df87d..b8bfda73a4 100644
--- a/packages/backend/src/server/api/endpoints/admin/announcements/create.ts
+++ b/packages/backend/src/server/api/endpoints/admin/announcements/create.ts
@@ -55,7 +55,7 @@ export const paramDef = {
 	properties: {
 		title: { type: 'string', minLength: 1 },
 		text: { type: 'string', minLength: 1 },
-		imageUrl: { type: 'string', nullable: true, minLength: 1 },
+		imageUrl: { type: 'string', nullable: true, minLength: 0 },
 		icon: { type: 'string', enum: ['info', 'warning', 'error', 'success'], default: 'info' },
 		display: { type: 'string', enum: ['normal', 'banner', 'dialog'], default: 'normal' },
 		forExistingUsers: { type: 'boolean', default: false },
@@ -76,7 +76,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				updatedAt: null,
 				title: ps.title,
 				text: ps.text,
-				imageUrl: ps.imageUrl,
+				/* 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,