1. An authentication device comprising: a processor executing instructions to: compare multiple types of biological information acquired from a target person of identity verification as acquired biological information with the same type of registered biological information preliminarily registered and calculate an authentication score representing a degree of similarity between the acquired biological information and the registered biological information, for each type of the acquired biological information; calculate, for each type of the acquired biological information, a probability that the target person of identity verification is fraudulently pretending to be the registered person who registered the registered biological information as an identity fraud probability, by utilizing the calculated authentication score; and carry out at least one of a determination whether the target person of identity verification is the registered person and a determination whether the target person of identity verification is fraudulently pretending to be the registered person, utilizing the identity fraud probability.