สอบถามเกี่ยวกับการ Login User หน่อยครับ ไม่ยอมเปลี่ยนหน้า มันฟ้องตลอดเลย

<form method='post' action='chk_user.php'>
  <table width="800" border="0" cellpadding="8" cellspacing="0">
    <tr>
      <td align="right" bgcolor="#FFFFFF"><strong class="font-gray-12px">USERNAME : </strong></td>
      <td align="left" bgcolor="#FFFFFF"><input type="text" name="username" style="width:300px;"></td>
    </tr>
    <tr>
      <td align="right" bgcolor="#FFFFFF"><strong class="font-gray-12px">PASSWORD : </strong></td>
      <td align="left" bgcolor="#FFFFFF"><input type="password" name="password" style="width:300px;"></td>
    </tr>
    <tr>
      <td align="right" bgcolor="#FFFFFF">&nbsp;</td>
      <td width="526" align="left" bgcolor="#FFFFFF"><label>
        <input type="submit" name="Submit" value="SUBMIT" style="background-color: #000; width:70px; height:25px; color:#fff; font-weight:bold; border:1px solid #666;">
        <input type="reset" name="Submit2" value="CANCLE" style="background-color:  #000; width:70px; height:25px; color:#fff; font-weight:bold; border:1px solid #666;">
      </label></td>
    </tr>
  </table>
	</form>

<?php
session_start();

include "connect.php"; 
	$username=$_POST["username"];
	$password=$_POST["password"];

$sql = "select * from xxxxx where username='$username' and password='$password'";
$result=mysql_query($sql,$conn) ; 
$rs = mysql_fetch_array($result);

if(mysql_num_rows($result) != 0) 
{ 
	$_SESSION[sess_userid] = $session_id;
	$_SESSION[sess_username] = $username;
	$_SESSION[sess_add_id] =$rs["admin_id"];
	$_SESSION[sess_name] =$rs["admin_name"];
	$_SESSION[sess_type] =$rs["admin_type"];

	header("Location:index.php");
}
else	 
{
	header("Location:index.html");
}
?>

<?php
session_start();
	$sess_userid = $_SESSION[sess_userid];
	$sess_username = $_SESSION[sess_username];
	$sess_add_id = $_SESSION[sess_add_id];
	$sess_name = $_SESSION[sess_name];
	$sess_type = $_SESSION[sess_type];

	if($sess_userid <>$session_id or $sess_username=="")
	{
		header("Location:index.html");
		exit();
	}
include "connect.php";
?>

<?
$host="localhost";
$username="root";
$password="";
$dbname="xxxxx_db";
$conn=mysql_connect( $host,$username,$password) or die ("cannot connect Mysql");
mysql_select_db($dbname) or die("เชื่อมต่อฐานข้อมูลไม่ได้"); 
mysql_db_query("$dbname","SET NAMES utf8");
mysql_query("SET character_set_results =NULL");
?>

<?php
include "chk_login.php";
$mode = $_GET['mode'];
$Page = $_GET['Page'];
?>