jquery: can't get input value

jquery: can't get input value

I'm trying send my value to ajax but it seems empty parameter. Then I try
like:
<input type="text" id="IDtag"/>
<button id="check"></button>
$('#check').click(function(){
var IDnum = $("#IDtag").val();
alert (IDnum);
});
alert is null even I already fill some value into IDtag.



UPDATE