function smartRollover() {   
  if(document.getElementsByTagName) {   
    var images = document.getElementsByTagName("img");   
    for(var i=0; i < images.length; i++) {   
      if(images[i].getAttribute("src").match(/_off\./)) {
        fileName = new Array(images[i].getAttribute("src").replace("_off.", "_on."));
        preImages = new Array();
        for (j=0; j<fileName.length; j++) {
          preImages[j] = new Image();
          preImages[j].src = fileName[j]; //「_on」の画像をプリロード
        }
          images[i].onmouseover = function() {   
            this.setAttribute("src", this.getAttribute("src").replace("_off.", "_on.")); //マウスオーバーで_off→_on
        }   
          images[i].onmouseout = function() {   
            this.setAttribute("src", this.getAttribute("src").replace("_on.", "_off.")); //マウスが離れたら_on→_off
        }   
      }   
    }   
  }   
}   
if(window.addEventListener) {   
    window.addEventListener("load", smartRollover, false); //実行
}   
else if(window.attachEvent) {   
    window.attachEvent("onload", smartRollover);   
}