socket.ioでのチャットアプリ実装

node.jsとsocket.ioでチャットアプリを作成しようとしているのですが、以下のようなエラーが出てしまい、nodeでサーバーを立ち上げることができません。どのように対処すればよろしいのでしょうか？
ポート番号やサーバー部分は変更してあります。

throw er; // Unhandled 'error' event
^

Error: listen EADDRINUSE :::3000
at Server.setupListenHandle [as _listen2] (net.js:1345:14)
at listenInCluster (net.js:1386:12)
at Server.listen (net.js:1474:7)
at Object.<anonymous> (/home/ok-dev/nodeapp/app.js:7:4)
at Module._compile (module.js:649:30)
at Object.Module._extensions..js (module.js:660:10)
at Module.load (module.js:561:32)
at tryModuleLoad (module.js:501:12)
at Function.Module._load (module.js:493:3)
at Function.Module.runMain (module.js:690:10)
Emitted 'error' event at:
at emitErrorNT (net.js:1365:8)
at process._tickCallback (internal/process/next_tick.js:114:19)
at Function.Module.runMain (module.js:692:11)
at startup (bootstrap_node.js:194:16)
at bootstrap_node.js:666:3

js
1var fs = require("fs");
2var server = require("http").createServer(function(req, res) {
3     res.writeHead(200, {"Content-Type":"text/html"});
4     var output = fs.readFileSync("./index.html", "utf-8");
5     res.end(output);
6}).listen(3000);
7var io = require("socket.io").listen(server);
8
9var userHash = {};
10
11io.sockets.on("connection", function (socket) {
12  socket.on("connected", function (name) {
13    var msg = name + "が入室しました";
14    userHash[socket.id] = name;
15    io.sockets.emit("publish", {value: msg});
16  });
17
18  socket.on("publish", function (data) {
19    io.sockets.emit("publish", {value:data.value});
20  });
21
22  socket.on("disconnect", function () {
23    if (userHash[socket.id]) {
24      var msg = userHash[socket.id] + "が退出しました";
25      delete userHash[socket.id];
26      io.sockets.emit("publish", {value: msg});
27    }
28  });
29});

html
1<html>
2<head>
3  <meta charset="UTF-8">
4  <title>サンプルチャット</title>
5</head>
6<body>
7  <input type="text" id="msg_input" style="width:200px;" />
8  <button onclick="publishMessage();">投稿</button>
9  <div id="msg"></div>
10  <script src="/socket.io/socket.io.js"></script>
11  <script type="text/javascript">
12    var socketio = io.connect('http://123.456.789.012:3000/');
13
14    socketio.on("connected", function(name) {});
15    socketio.on("publish", function (data) { addMessage(data.value); });
16    socketio.on("disconnect", function () {});
17
18    function start(name) {
19      socketio.emit("connected", name);
20    }
21
22    function publishMessage() {
23      var textInput = document.getElementById('msg_input');
24      var msg = "[" + myName + "] " + textInput.value;
25      socketio.emit("publish", {value: msg});
26      textInput.value = '';
27    }
28
29    function addMessage (msg) {
30      var domMeg = document.createElement('div');
31      domMeg.innerHTML = new Date().toLocaleTimeString() + ' ' + msg;
32      msgArea.appendChild(domMeg);
33    }
34
35    var msgArea = document.getElementById("msg");
36    var myName = Math.floor(Math.random()*100) + "さん";
37    addMessage("貴方は" + myName + "として入室しました");
38    start(myName);
39  </script>
40</body>
41</html>