function preloadImages()
    {
      preload_image_object = new Image();
      // set image url
      image_url = new Array();
      image_url[0] = "images/logo_home.gif";
      image_url[1] = "images/sub_lefttmenu_bt_01_over.gif";
      image_url[2] = "images/sub_lefttmenu_bt_02_over.gif";
      image_url[3] = "images/sub_lefttmenu_bt_03_over.gif";
      image_url[4] = "images/sub_lefttmenu_bt_04_over.gif";
      image_url[5] = "images/sub_lefttmenu_bt_05_over.gif";
      image_url[6] = "images/menu_bt_01_over.gif";
      image_url[6] = "images/menu_bt_02_over.gif";
      image_url[6] = "images/menu_bt_03_over.gif";
      image_url[6] = "images/menu_bt_04_over.gif";
      image_url[6] = "images/menu_bt_05_over.gif";
      image_url[6] = "images/menu_bt_06_over.gif";

       var i = 0;
       for(i=0; i<=3; i++) 
         preload_image_object.src = image_url[i];
    }