/* 画像の先行読み込み */
if(document.images) {
 var imgs = new Array();
 imgs[0] = new Image();  imgs[0].src = "top_material/mhb_img.jpg"; //画像の設定
 imgs[1] = new Image();  imgs[1].src = "top_material/mhb_img_on.jpg";
 imgs[2] = new Image();  imgs[2].src = "top_material/msh_img.jpg";
 imgs[3] = new Image();  imgs[3].src = "top_material/msh_img_on.jpg";
}

function chgimg(id, imgname) {
 if(document.images) {
  document.images[id].src = (imgs[imgname]) ? imgs[imgname].src : imgname;
 }
}