function initRollOvers() {
    if (!document.getElementById) {
        return;
    }

    var preLoads = new Array();
    var allImages = document.getElementsByTagName('img');

    for (var i = 0; i < allImages.length; i++) {
        if (allImages[i].className == 'rollover') {
            var src = allImages[i].getAttribute('src');
            var ftype = src.substring(src.lastIndexOf('.'), src.length);
            var oSrc = src.replace(ftype, '_on' + ftype);

            //-- スワップ元、スワップ先画像の登録
            allImages[i].setAttribute('pSrc', src);
            allImages[i].setAttribute('oSrc', oSrc);

            //-- イメージのプリロード
            preLoads[i] = new Image();
            preLoads[i].src = oSrc;

            //-- イベントの設定
            allImages[i].onmouseover = function () {
                this.setAttribute('src', this.getAttribute('oSrc'));
            }
            allImages[i].onmouseout = function () {
                this.setAttribute('src', this.getAttribute('pSrc'));
            }
        }
    }
}

function addOnload(func) {
    if (typeof window.addEventListener != "undefined") {
        window.addEventListener("load", func, false);
    } else if (typeof window.attachEvent != "undefined") {
        window.attachEvent("onload", func);
    } else {
        if (window.onload != null) {
            var oldOnload = window.onload;
            window.onload = function (e) {
                oldOnload(e);
                window[func]();
            };
        } else
            window.onload = func;
    }
}
addOnload(initRollOvers);