1. An image processor comprising: a visual point conversion section respectively converting images photographed by an n-number of photography apparatuses (where n is an integer of 2 or larger) into bird's-eye view images as viewed from a virtual visual point; and an image synthesis section synthesizing the obtained bird's-eye view images to generate a synthetic bird's-eye view image, the image processor generating a video signal for displaying the synthetic bird's-eye view image on a display device, wherein the image synthesis section compares, between a plurality of the bird's-eye view images, images of a common region where the plurality of the bird's-eye view images overlap upon the synthesis to thereby specify a difference region between the plurality of the bird's-eye view images in the common region, wherein weight values corresponding to respective pixel positions of the image in the common region are set for each of the plurality of the bird's-eye view images, wherein part or all of the set weight values differ between the plurality of the bird's-eye view images, and wherein the image synthesis section sums up the weight values corresponding to pixels included in the difference region to thereby calculate a summed value for each of the plurality of the bird's-eye view images, selects, based on the summed values, one bird's-eye view image from among the plurality of the bird's-eye view images as an image to be adopted, and adopts, as an image of the common region in the synthetic bird's-eye view image, the image of the common region in the image to be adopted.