1. A region specification method of specifying a subject region including a subject from a subject existing image in which a background and the subject exist, the region specification method comprising: specifying a pixel group corresponding to the subject region of the subject existing image based on difference information between a pixel of the subject existing image and a corresponding pixel of a background image which is located at a position in the background image corresponding to a position of the pixel in the subject existing image; specifying a bottom image boundary of an image boundary of the subject existing image displayed on a predetermined display region; and changing each pixel of a region enclosed by the specified pixel group and the specified bottom image boundary, and which was not specified as a pixel of the pixel group corresponding to the subject region, so as to be specified as a pixel of the specified pixel group corresponding to the subject region.