1. An image processing method, comprising: (a) computing difference values between a boundary equivalent to an outer edge of a synthesis target region corresponding to an N th frame of a video image, and a boundary equivalent to an outer edge of a synthesis target region corresponding to an N−1 th frame of the video image; (b) generating a difference image such that, whilst pixel values at a boundary of the difference image maintain the computed difference values, a region inside the boundary of the difference image matches a specific gradient condition; and (c) deriving, by a processor, as a synthesized result image for the N th frame, an image that is the generated difference image added to a synthesized result image derived for the N−1 th frame that is equivalent to a result of synthesizing a synthesis image at the synthesis target region in the N−1 th frame, and substituting the synthesized result image derived for the N th frame for an image of the synthesis target region in the N th frame.