diff --git a/NaiveSharp/Program.cs b/NaiveSharp/Program.cs
index a58ecad..f48252b 100644
--- a/NaiveSharp/Program.cs
+++ b/NaiveSharp/Program.cs
@@ -1,4 +1,7 @@
 using System;
+using System.Diagnostics;
+using System.Reflection;
+using System.Runtime.InteropServices;
 using System.Windows.Forms;
 
 namespace NaiveSharp
@@ -10,7 +13,46 @@ static void Main()
         {
             Application.EnableVisualStyles();
             Application.SetCompatibleTextRenderingDefault(false);
-            Application.Run(new MainWindow());
+            Process instance = RunningInstance();
+            if (instance == null)
+            {
+                Application.Run(new MainWindow());
+            }
+            else
+            {
+                MessageBox.Show("Naive# has been running.", "Info", MessageBoxButtons.OK, MessageBoxIcon.Information);
+                HandleRunningInstance(instance);
+            }
         }
+
+        public static Process RunningInstance()
+        {
+            Process current = Process.GetCurrentProcess();
+            Process[] processes = Process.GetProcessesByName(current.ProcessName);
+            foreach (Process process in processes)
+            {
+                if (process.Id != current.Id)
+                {
+                    if (Assembly.GetExecutingAssembly().Location.Replace("/", "\\") == current.MainModule.FileName)
+                    {
+                        return process;
+                    }
+                }
+            }
+            return null;
+        }
+
+        public static void HandleRunningInstance(Process instance)
+        {
+            ShowWindow(instance.MainWindowHandle, WS_SHOWNORMAL);
+            SetForegroundWindow(instance.MainWindowHandle);
+        }
+
+        [DllImport("User32.dll")]
+        private static extern bool ShowWindow(IntPtr hWnd, int cmdShow);
+
+        [DllImport("User32.dll")]
+        private static extern bool SetForegroundWindow(IntPtr hWnd);
+        private const int WS_SHOWNORMAL = 1;
     }
 }