var imgDir="http://www.stepon.co.jp/s_images/";
        bt01=new Image;bt01.src=imgDir + "bt07a.gif";
        bt01_on=new Image;bt01_on.src=imgDir + "bt07b.gif";
        bt02=new Image;bt02.src=imgDir + "bt08a.gif";
        bt02_on=new Image;bt02_on.src=imgDir + "bt08b.gif";
        bt03=new Image;bt03.src=imgDir + "bt09a.gif";
        bt03_on=new Image;bt03_on.src=imgDir + "bt09b.gif";
        bt04=new Image;bt04.src=imgDir + "bt10a.gif";
        bt04_on=new Image;bt04_on.src=imgDir + "bt10b.gif";
        bt05=new Image;bt05.src=imgDir + "bt11a.gif";
        bt05_on=new Image;bt05_on.src=imgDir + "bt11b.gif";
        bt06=new Image;bt06.src=imgDir + "bt12a.gif";
        bt06_on=new Image;bt06_on.src=imgDir + "bt12b.gif";

function chgImg(name,chgImg){
        if(document.images){document.images[name].src=eval(chgImg+".src");}
}