1. A region specification method of specifying a subject region including a subject from a subject exiting image in which a background and the subject exist, the region specification method comprising: calculating an image boundary pixel number of an image boundary of the subject existing image constituting an edge part of each of divided regions into which the subject existing image is divided by a borderline defined on the subject existing image; specifying, from the divided regions, a reference region having a pixel percentage equal to or more than a predetermined percentage, the pixel percentage indicating the calculated image boundary pixel number in a total pixel number of the edge part; and specifying the subject region from the divided regions of the subject existing image by taking the reference region specified in the specifying the reference region as a reference.