diff --git a/CHANGELOG.md b/CHANGELOG.md
index 89e52f5cc8..fb2212ebd2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 ### Client
 - Fix: RSSティッカーウィジェットが正しく動作しない問題を修正
 - Fix: エラー画像が横に引き伸ばされてしまう問題に対応
+- Enhance: /flushページでサイトキャッシュをクリアできるようになりました
 
 ### Server
 -
diff --git a/packages/backend/src/server/web/ClientServerService.ts b/packages/backend/src/server/web/ClientServerService.ts
index b515a0c0c8..3cd83efa1a 100644
--- a/packages/backend/src/server/web/ClientServerService.ts
+++ b/packages/backend/src/server/web/ClientServerService.ts
@@ -201,6 +201,8 @@ export class ClientServerService {
 
 	@bindThis
 	public createServer(fastify: FastifyInstance, options: FastifyPluginOptions, done: (err?: Error) => void) {
+		const configUrl = new URL(this.config.url);
+
 		fastify.register(fastifyView, {
 			root: _dirname + '/views',
 			engine: {
@@ -239,7 +241,6 @@ export class ClientServerService {
 				done();
 			});
 		} else {
-			const configUrl = new URL(this.config.url);
 			const urlOriginWithoutPort = configUrl.origin.replace(/:\d+$/, '');
 
 			const port = (process.env.VITE_PORT ?? '5173');
@@ -887,6 +888,22 @@ export class ClientServerService {
 			[, ...target.split('/').filter(x => x), ...source.split('/').filter(x => x).splice(depth)].join('/');
 
 		fastify.get('/flush', async (request, reply) => {
+			let sendHeader = true;
+
+			if (request.headers['origin']) {
+				const originURL = new URL(request.headers['origin']);
+				if (originURL.protocol !== 'https:') { // Clear-Site-Data only supports https
+					sendHeader = false;
+				}
+				if (originURL.host !== configUrl.host) {
+					sendHeader = false;
+				}
+			}
+
+			if (sendHeader) {
+				reply.header('Clear-Site-Data', '"*"');
+			}
+			reply.header('Set-Cookie', 'http-flush-failed=1; Path=/flush; Max-Age=60');
 			return await reply.view('flush');
 		});
 
diff --git a/packages/backend/src/server/web/views/flush.pug b/packages/backend/src/server/web/views/flush.pug
index a73a45212f..7884495d08 100644
--- a/packages/backend/src/server/web/views/flush.pug
+++ b/packages/backend/src/server/web/views/flush.pug
@@ -6,41 +6,45 @@ html
 		const msg = document.getElementById('msg');
 		const successText = `\nSuccess Flush! <a href="/">Back to Misskey</a>\n成功しました。<a href="/">Misskeyを開き直してください。</a>`;
 
-		message('Start flushing.');
+		if (!document.cookie) {
+			message('Your site data is fully cleared by your browser.');
+			message(successText);
+		} else {
+			message('Your browser does not support Clear-Site-Data header. Start opportunistic flushing.');
+			(async function() {
+				try {
+					localStorage.clear();
+					message('localStorage cleared.');
 
-		(async function() {
-			try {
-				localStorage.clear();
-				message('localStorage cleared.');
+					const idbPromises = ['MisskeyClient', 'keyval-store'].map((name, i, arr) => new Promise((res, rej) => {
+						const delidb = indexedDB.deleteDatabase(name);
+						delidb.onsuccess = () => res(message(`indexedDB "${name}" cleared. (${i + 1}/${arr.length})`));
+						delidb.onerror = e => rej(e)
+					}));
 
-				const idbPromises = ['MisskeyClient', 'keyval-store'].map((name, i, arr) => new Promise((res, rej) => {
-					const delidb = indexedDB.deleteDatabase(name);
-					delidb.onsuccess = () => res(message(`indexedDB "${name}" cleared. (${i + 1}/${arr.length})`));
-					delidb.onerror = e => rej(e)
-				}));
+					await Promise.all(idbPromises);
 
-				await Promise.all(idbPromises);
+					if (navigator.serviceWorker.controller) {
+						navigator.serviceWorker.controller.postMessage('clear');
+						await navigator.serviceWorker.getRegistrations()
+							.then(registrations => {
+								return Promise.all(registrations.map(registration => registration.unregister()));
+							})
+							.catch(e => { throw new Error(e) });
+					}
 
-				if (navigator.serviceWorker.controller) {
-					navigator.serviceWorker.controller.postMessage('clear');
-					await navigator.serviceWorker.getRegistrations()
-						.then(registrations => {
-							return Promise.all(registrations.map(registration => registration.unregister()));
-						})
-						.catch(e => { throw new Error(e) });
+					message(successText);
+				} catch (e) {
+					message(`\n${e}\n\nFlush Failed. <a href="/flush">Please retry.</a>\n失敗しました。<a href="/flush">もう一度試してみてください。</a>`);
+					message(`\nIf you retry more than 3 times, try manually clearing the browser cache or contact to instance admin.\n3回以上試しても失敗する場合、ブラウザのキャッシュを手動で消去し、それでもだめならインスタンス管理者に連絡してみてください。\n`)
+
+					console.error(e);
+					setTimeout(() => {
+						location = '/';
+					}, 10000)
 				}
-
-				message(successText);
-			} catch (e) {
-				message(`\n${e}\n\nFlush Failed. <a href="/flush">Please retry.</a>\n失敗しました。<a href="/flush">もう一度試してみてください。</a>`);
-				message(`\nIf you retry more than 3 times, clear the browser cache or contact to instance admin.\n3回以上試しても失敗する場合、ブラウザのキャッシュを消去し、それでもだめならインスタンス管理者に連絡してみてください。\n`)
-
-				console.error(e);
-				setTimeout(() => {
-					location = '/';
-				}, 10000)
-			}
-		})();
+			})();
+		}
 
 		function message(text) {
 			msg.insertAdjacentHTML('beforeend', `<p>[${(new Date()).toString()}] ${text.replace(/\n/g,'<br>')}</p>`)