diff --git a/docs/style.css b/docs/style.css
index ea6fbcd..ec7cdad 100644
--- a/docs/style.css
+++ b/docs/style.css
@@ -117,6 +117,12 @@ header {
     content: " (未選択)";
 }
 
+#unsupported {
+  max-width: 640px;
+  margin: 2ex auto 0;
+  color: red;
+}
+
 .bgblack-fade {
     background-color: black;
     transition-property: background-color;
diff --git a/docs/xsystem35.html b/docs/xsystem35.html
index 282d53d..06708fb 100644
--- a/docs/xsystem35.html
+++ b/docs/xsystem35.html
@@ -48,6 +48,10 @@
       <div class="emscripten" id="status"></div>
       <canvas class="emscripten" id="canvas" oncontextmenu="event.preventDefault()"></canvas>
     </div>
+
+    <div id="unsupported" hidden>
+      このブラウザでは利用できません。
+    </div>
   </div>
 
   <!-- Settings -->
@@ -110,5 +114,11 @@
   </script>
   <script src="shell.js"></script>
   <script src="//cdnjs.cloudflare.com/ajax/libs/jszip/3.1.3/jszip.min.js" async></script>
+  <script>
+    if (xsystem35.shell === undefined) {
+      loader.hidden = true;
+      unsupported.hidden = false;
+    }
+  </script>
   </body>
 </html>