js循环显示对象id问题

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>无标题页</title>
</head>
<body>
<div id="content">
<div id="Div"></div>
<div id="Div1"></div>
<div id="Div2"></div>
<div id="Div3"></div>
<div id="Div4"></div>
<div id="Div5"></div>
</div>
<script>
var obj=document.getElementById("content").getElementsByTagName("div");
for(var i=0;i<obj.length;i++)
{
alert(obj[i].id);
}
</script>
</body>
</html>