var img1on = new Image();
img1on.src = "images/img_ga_ov.gif"; 
var img1off = new Image();
img1off.src = "images/img_ga.gif"; 

var img2on = new Image();
img2on.src = "images/img_jyukusei_ov.gif"; 
var img2off = new Image();
img2off.src = "images/img_jyukusei.gif"; 

function On(name) { 
if (document.images) { 
document.images[name].src = eval(name + 'on.src'); 
} 
} 

function Off(name) { 
if (document.images) { 
document.images[name].src = eval(name + 'off.src'); 
} 
}