diff --git a/contents/_templates/links.html b/contents/_templates/links.html
new file mode 100644
index 0000000..c060a39
--- /dev/null
+++ b/contents/_templates/links.html
@@ -0,0 +1,7 @@
+{# Sphinx sidebar template: "show source" link. #}
+<div role="note" aria-label="source link">
+  <h3>{{ _('External links') }}</h3>
+  <ul class="this-page-menu">
+    <li><a href="https://github.com/atsphinx/sandbox/" rel="noreferrer" target="_blank">GitHub</a></li>
+  </ul>
+</div>
diff --git a/contents/conf.py b/contents/conf.py
index 6d99409..602a4b1 100644
--- a/contents/conf.py
+++ b/contents/conf.py
@@ -32,3 +32,12 @@
 
 html_theme = "bizstyle"
 html_static_path = ["_static"]
+html_sidebars = {
+    "**": [
+        "localtoc.html",
+        "relations.html",
+        "sourcelink.html",
+        "searchbox.html",
+        "links.html",
+    ],
+}