javascript function for validate input in only decimal format

<form name="form" method="post" onsubmit="return validate();">
Enter Decimal Number:<input type="text" name="num"> <input
type="submit" value="Check">
</form>

<script type="text/javascript" language="javascript">
function isDecimal(str){
if(isNaN(str) || str.indexOf(".")<0){
alert("Invalid");
}
else{
str=parseFloat(str);
alert ("Entered number is decimal")
}
}
function validate(){
var dec=document.form.num.value;
if (dec == ""){
alert("Please enter.");
form.num.focus();
return false;
}
if (isDecimal(dec)==false){
num="";
form.num.focus();
return false;
}
return true;
}
</script>