onsubmit not working

My onsubmit is not working. Code here------------

<script>
function check(){
if(document.forms["check"]["f_name"].value==NULL || document.forms["check"]["f_name"].value==""){
getElementById('showpad').innerHTML="Please enter your name";
return false;
}
else{
getElementById('showpad').innerHTML="Thank You !";
return true;
}
}
</script>

<form id="check" name="check" onsubmit="return check()" method="post">
<input name="f_name" id="f_name" type="text" />
<p id="showpad"></p>
<input type="submit" value="submit" />
</form>