An authentication device includes: a vein information extraction section that extracts a piece of vein information representing a vein from an image containing a vein of a finger; an extraction section that extracts a piece of information representing the position of the finger's outline at an intermediate stage of the process of extracting the piece of vein information; and a determination section that determines, based on the degree of similarity between the piece of information and a piece of information associated with a piece of registration-target vein information, a verification target to be compared with the piece of vein information.