diff --git a/websocket-sharp/Server/WebSocketSessionManager.cs b/websocket-sharp/Server/WebSocketSessionManager.cs
index c5be3ec1..ab3b2739 100644
--- a/websocket-sharp/Server/WebSocketSessionManager.cs
+++ b/websocket-sharp/Server/WebSocketSessionManager.cs
@@ -151,15 +151,31 @@ namespace WebSocketSharp.Server
     }
 
     /// <summary>
-    /// Gets the session with the specified <paramref name="id"/>.
+    /// Gets the session instance with the specified <paramref name="id"/>.
     /// </summary>
     /// <value>
-    /// A <see cref="IWebSocketSession"/> instance that provides the access to
-    /// the information in the session, or <see langword="null"/> if it's not found.
+    ///   <para>
+    ///   A <see cref="IWebSocketSession"/> instance or
+    ///   <see langword="null"/> if not found.
+    ///   </para>
+    ///   <para>
+    ///   That session instance provides the function to
+    ///   access the information in the session.
+    ///   </para>
     /// </value>
     /// <param name="id">
-    /// A <see cref="string"/> that represents the ID of the session to find.
+    /// A <see cref="string"/> that represents the ID of
+    /// the session to find.
     /// </param>
+    /// <exception cref="InvalidOperationException">
+    /// The current state of the manager is not Start.
+    /// </exception>
+    /// <exception cref="ArgumentNullException">
+    /// <paramref name="id"/> is <see langword="null"/>.
+    /// </exception>
+    /// <exception cref="ArgumentException">
+    /// <paramref name="id"/> is an empty string.
+    /// </exception>
     public IWebSocketSession this[string id] {
       get {
         if (_state != ServerState.Start) {