diff --git a/docs/index.html b/docs/index.html
index bd5667e..01c598e 100644
--- a/docs/index.html
+++ b/docs/index.html
@@ -8,6 +8,15 @@
   <title>NES.css - NES-style CSS Framework</title>
   <link href="https://rawgit.com/BcRikko/NES.css/master/css/nes.css" rel="stylesheet">
   <link href="./style.css" rel="stylesheet">
+  <script async src="https://www.googletagmanager.com/gtag/js?id=UA-41640153-4"></script>
+  <script>
+    window.dataLayer = window.dataLayer || [];
+    function gtag() { dataLayer.push(arguments); }
+    gtag('js', new Date());
+
+    gtag('config', 'UA-41640153-4');
+  </script>
+
 </head>
 
 <body>
