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();
        preImages = new Image();
        preImages.src = fileName; // 画像をプリロード
      }

      if(images[i].getAttribute("src").match("_off."))
      {
        images[i].onmouseover = function() {
          this.setAttribute("src", this.getAttribute("src").replace("_off.", "_on."));
        }
        images[i].onmouseout = function() {
          this.setAttribute("src", this.getAttribute("src").replace("_on.", "_off."));
        }
      }
    }
  }
}
if(window.addEventListener) {
  window.addEventListener("load", smartRollover, false);
}
else if(window.attachEvent) {
  window.attachEvent("onload", smartRollover);
}