function random(){

url = new Array(4); 
img = new Array(url.length); 

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

url[0]="http://www.hirase-gakki.com/g3/120011.html";
img[0]="http://hirase-gakki.com/ban/gtako.gif";

url[1]="http://www.hirase-gakki.com/g3/380008.html";
img[1]="http://hirase-gakki.com/ban/03syuugi.jpg";

url[2]="http://www.hirase-gakki.com/cgi-bin/site_search.cgi?mode=and&key=%83%84%83b%83%5E%81%5B%83%7D%83%93+%83V%83%8A%81%5B%83Y";
img[2]="http://hirase-gakki.com/ban/04yatta.jpg";

url[3]="http://www.codomo-melody.com/g3/10007.html";
img[3]="http://www.codomo-melody.com/ban/01snare.jpg";

/*これ以降は、3,4,5と数字を増やしてください。*/

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

}