1. An image display device comprising: a region-of-interest setting section that sets two regions-of-interest respectively for two medical images representing a photographed subject; a difference image creating section that creates a difference image representing a difference between two image portions of the two regions-of-interest set by the region-of-interest setting section; and an image displaying section that retrieves one image portion of the two image portions from one medical image of the two medical images which includes the one image portion, and that displays on a same display screen the difference image as well as the two medical images arranged side by side with the difference image, while displaying the retrieved one image portion in such a manner that the retrieved one image portion overlaps with the other medical image of the two medical images which includes the other image portion of the two image portions and does not include the retrieved one image portion, the retrieved one image portion overlapping with the other medical image at a position next to the other image portion on the other medical image.