<form name= form>
<input class="field" name=money onkeyup="number_format(this)">
</form>

<script language="javascript">
function number_format(f) {
 var val = f.value;
 var len = val.length;
    var number_format1 = "", number_format2 = "";
 var c = 0;
 
 if(val.charCodeAt(len-1)<48 || val.charCodeAt(len-1)>57) {
  alert("숫자만 입력해주세요");
  f.value = val.substr(0, (len-1));
 }else{
     if(len > 3) {   
         for(i = 0 ; i < len; i++){
    one = val.charAt(i)
    if(one != ",") number_format1 += one;
         }
   var number_format1_len = number_format1.length;
   var in_c = number_format1_len%3;
   if(!in_c) in_c = 3;
   
   for(i = 0 ; i < number_format1_len; i++){
    number_format2_one = number_format1.charAt(i)
    if(i == in_c){
     number_format2 += ",";
     in_c = 3+in_c;
    }
    number_format2 += number_format2_one;
         }
         f.value = number_format2;
     }
 }
}
</script>