1. An image generating method comprising: a first step of generating a small region panorama image group which is an image group of N images which include a shooting range common to each other, the N images being composed of a plurality of common partially overlapped regions and having independency from each other in each of the partially overlapped regions, wherein N is an integer larger than 2; and a second step of generating, for a target image obtained by shooting at least a part of the shooting range, a predicted background image in which light source environment is the same as that of the target image with the use of the target image and the small region panorama image group.