function focus_move(no){
var len=document.getElementsByName('vip_num'+no)[0].value.length;
if(len==4){
try{
document.getElementsByName('vip_num'+(no+1))[0].focus();
} catch(e){}
}
}
//숫자만 입력하게//
function ncheck(num){
var chkflag=true;
for(var i =0;i<num.length;i++){
c=num.charAt(i);
if(!(c>='0' && c<='9')) {
chkflag=false;
break;
}
}
return chkflag;
}
function numchk(v,n){
if(!ncheck(v)){
alert('숫자만 입력하세요');
var temp=eval("document.sform."+n); //form이름이 form1이더군요.. 1추가했구요..
temp.value='';
temp.focus();
}
}
<td class="pd_l10 pd_t10 pd_b10 bd_tb bg_f5" width="510"><input name="vip_num1" type="text" class="s" size="10" onkeyup="numchk(this.value,this.name);focus_move(1)" /> - <input name="vip_num2" type="text" class="s" size="10" onkeyup="numchk(this.value,this.name);focus_move(2)" /> - <input name="vip_num3" type="text" class="s" size="10" onkeyup="numchk(this.value,this.name);focus_move(3)" /></td>