<SCRIPT Language="Javascript">
<!-- 감춤
function changeSub(){
  if (formName.big.value=='대분류1'){
    SubMenus=new Array('중분류10','중분류11','중분류12','중분류13','중분류14','중분류15')
  } else if (formName.big.value=='대분류2'){
    SubMenus=new Array('중분류20','중분류21','중분류22','중분류23','중분류24','중분류25')
  } else if (formName.big.value=='대분류3'){
    SubMenus=new Array('중분류30','중분류31','중분류32','중분류33','중분류34','중분류35')
  }
  for (i=formName.medium.options.length;i >=0;i--){
    formName.medium.remove(i);
  }
  for (i=0;i < SubMenus.length; i++){
    var oNewNode=document.createElement('OPTION');
    formName.medium.appendChild(oNewNode);
    oNewNode.innerText=SubMenus[i];
  }
}
// 감춤 끝  -->
</SCRIPT>

<FORM name="formName">
<SELECT name="big" onchange="changeSub()">
 <OPTGROUP label="대분류">
 <OPTION value="대분류1" selected>대분류1
 <OPTION value="대분류2">대분류2
 <OPTION value="대분류3">대분류3
</SELECT>
<SELECT name="medium">
 <OPTGROUP label="중분류">
 <OPTION>중분류
</SELECT>
</FORM>