1. An authentication apparatus comprising: a first determination section that determines whether a condition for determining that there is a possibility of an erroneous determination is satisfied based on information indicating the similarity between the shape of a biological part included in a biological image to be authenticated and the shape of a biological part included in a registration biological image; a change section that changes an aspect ratio of the biological image to be authenticated and registration biological image in the case where said condition is satisfied; and a second determination section that determines whether a person to be authenticated is a registrant based on the similarity between the shape of a biological part included in a biological image to be authenticated whose aspect ratio has been changed and the shape of a biological part included in a registration biological image whose aspect ratio has been changed.