function preload_image() {
    _preload = new Array();
    for (var i = 0; i < 16; i++) {
        var n = i + 1;
        n = "00" + n;
        n = "image/ind_btn" + n.substring(n.length - 3) + "_over.jpg";
        _preload[i] = new Image();
        _preload[i].src = n;
    }
}
preload_image();