When a plurality of images are to be overlapped and synthesized, coincidence degrees between the plurality of images in an area near the boundary of a common area are calculated, and the plurality of images are overlapped and synthesized so that an area near the boundary having a high coincidence degree appears on the overlapped image. Thereby, it is possible to avoid the duplexing of the subject in the common area and unnatural ruptures on the boundary made by the overlapping while reducing the number of processes for the synthesizing.