1. An authentication device comprising: a physical central processing unit (CPU); a vein information extraction section, performed by the CPU, that extracts a piece of vein information representing a vein from an image containing a vein of a finger, the extraction of the vein information comprising a plurality of intermediate processes before completing the extraction at a final process; an extraction section, performed by the CPU, that extracts a first piece of information from a first one of the intermediate processes performed by the vein information extraction section to extract the piece of vein information, the first piece of information representing a first outline of the finger; and a determination section, performed by the CPU, that determines a verification target to be compared with the piece of vein information based on a degree of similarity between the first piece of information and a piece of extraction information extracted from the verification target, wherein the first piece of information is extracted from a predetermined partial area of the image such that the first outline of the finger is a partial portion of a full outline in the image, and represents positions of first pixels of the first outline with respect to second pixels at an edge of the predetermined partial area of the image.