function random(){

url = new Array(3); 
img = new Array(url.length); 

i = Math.floor(Math.random( ) * url.length);

url[0]="http://shoji-bb.com/shop/";
img[0]="img/left_ch.jpg";

url[1]="http://broadway-bb.com/";
img[1]="img/left_br.jpg";

url[2]="http://boston-bb.com/";
img[2]="img/left_bo.jpg";

document.write('<a href="'+url[i]+'" target="_blank">'); 
document.write('<img src="'+img[i]+'" border="0"></a>');

}