JS添加文本框的问题

<script language="javascript">                
function add()                
{
   option = new Array();
   proportion = new Array();            
   str='<table>';
   str=str+'<tr align=center valign=middle bgcolor=#FFFFFF>';
   str=str+'<td width="100%" height="25"><input type="text" name=proportion></td>';
   str=str+'<td width="100%" height="25"><input type="button" name=del onclick="this.parentNode.parentNode.parentNode.deleteRow(this.parentNode.parentNode.rowIndex)" value="删除"></td>';
   str=str+'</tr></table>';
   window.upid.innerHTML+=str+'';
}                
</script>
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head> 
<body>  
<form name="myform" onsubmit="javascript:return check()">  
   <table>  
       <tr>    
           <td> 
               <input name="Submit2" type="button" onclick="javascript:add()" value="增加">  
           </td>    
       </tr>    
       <tr>      
           <td>    
           <div id="upid"></div>  
           </td>    
       </tr> 
   </table>                
</form>                
</body>                
</html>