A computer-readable storage medium storing an image processing program that causes a computer to execute a process includes, synthesizing, for each of frames included in a video image and to be processed, a synthesis image with an image of a synthesis target region existing in the frame by repeatedly executing a calculation using a Poisson's equation on the basis of the image of synthesis target region existing in the frame to be processed and the synthesis image to be replaced with the image of the synthesis target region, and thereby calculating, from an initial value image, a synthesis result image corresponding to a result obtained by synthesizing the synthesis image with the synthesis target region so as to sequentially execute a synthesis process on the frames of the video image; and setting, for each of the frames to be processed, initial values by setting, as the initial value image.