An authentication device includes a last session information storage portion that stores data of a first registered person successfully authenticated in the last session, an input unit to which information of a second registered person successfully authenticated by another authentication device is input, an information acquisition unit that acquires identification information of a user via an information sensor, a registered person information storage portion that stores registered person information in which a plurality of registered persons including the first and second registered persons are associated with their registration identification information, an authentication order information storage portion that stores information about an authentication order of the registered persons, and an authentication unit that authenticates the user by comparing the identification information acquired from the information acquisition unit with the registration identification information stored in the registered person information storage portion according to the stored authentication order.