This authentication device includes: a volatile memory; a non-volatile memory which stores a plurality of electronic certificate files; a unit which refers to the non-volatile memory upon start-up, and which stores a hierarchical relationship between the plurality of electronic certificate files in the volatile memory; a unit for searching for a desired electronic certificate file based upon the hierarchical relationship between the plurality of electronic certificate files in the volatile memory; and an authentication unit which performs authentication using the electronic certificate file which has been found by the search unit.