function preload(imgs){
 for(var i = 0; i < imgs.length; i++){
   var imgObj = new Image();
   imgObj.src = imgs[i];
 }
}
preload(["img/b_concept.gif", "img/b_concept_on.gif",
, "img/b_course.gif", "img/b_course_on.gif"
, "img/b_gakkou.gif", "img/b_gakkou_on.gif"
, "img/b_open.gif", "img/b_open_on.gif"
, "img/b_schedule.gif", "img/b_schedule_on.gif"
, "img/b_special.gif", "img/b_special_on.gif"
, "img/b_submenu_line.gif"
]);