var imgMax  = 6; //セットする画像の枚数。
var imgRand = Math.floor(Math.random() * imgMax);

if(imgRand == 0)       document.write('<IMG src="ctr_img/ctr_rdm/ctr_idx/all_pht_0000.jpg" width=800 height=400 border=0>');

else if(imgRand == 1)  document.write('<IMG src="ctr_img/ctr_rdm/ctr_idx/all_pht_0001.jpg" width=800 height=400 border=0>');

else if(imgRand == 2)  document.write('<IMG src="ctr_img/ctr_rdm/ctr_idx/all_pht_2050.jpg" width=800 height=400 border=0>');

else if(imgRand == 3)  document.write('<IMG src="ctr_img/ctr_rdm/ctr_idx/all_pht_2283.jpg" width=800 height=400 border=0>');

else if(imgRand == 4)  document.write('<IMG src="ctr_img/ctr_rdm/ctr_idx/all_pht_2297.jpg" width=800 height=400 border=0>');

else if(imgRand == 5)  document.write('<IMG src="ctr_img/ctr_rdm/ctr_idx/all_pht_2302.jpg" width=800 height=400 border=0>');