diff --git a/Assets/UniRx.Async/DiagnosticsExtensions.cs b/Assets/UniRx.Async/Internal/DiagnosticsExtensions.cs
similarity index 98%
rename from Assets/UniRx.Async/DiagnosticsExtensions.cs
rename to Assets/UniRx.Async/Internal/DiagnosticsExtensions.cs
index d0dfd33..77fd2e1 100644
--- a/Assets/UniRx.Async/DiagnosticsExtensions.cs
+++ b/Assets/UniRx.Async/Internal/DiagnosticsExtensions.cs
@@ -15,10 +15,9 @@ using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 using UnityEngine;
 
-namespace UniRx.Async
+namespace UniRx.Async.Internal
 {
-    // TODO: Internal.
-    public static class DiagnosticsExtensions
+    internal static class DiagnosticsExtensions
     {
         static bool displayFilenames = true;
 
diff --git a/Assets/UniRx.Async/DiagnosticsExtensions.cs.meta b/Assets/UniRx.Async/Internal/DiagnosticsExtensions.cs.meta
similarity index 100%
rename from Assets/UniRx.Async/DiagnosticsExtensions.cs.meta
rename to Assets/UniRx.Async/Internal/DiagnosticsExtensions.cs.meta