function RandomImg(){ 
   Img=new Array(); 
   Img[0] =' <a href ="http://www.tsurushin.co.jp" target="_blank"><img src="/imgs/bnr_sinkin.gif" height="45" width="350" border="0"></a> '; 
   Img[1] =' <a href ="http://www.sky-a.co.jp" target="_blank"><img src="/imgs/sky_a.gif" height="45" width="350" border="0"></a> '; 
   var imgNum=Img.length; 
   var i=Math.floor(Math.random()*imgNum); 
   document.write(Img[i]); 
   } 

   RandomImg();