A device that ensures that one device acquires and updates a list used for judging whether the other device in communication is valid or invalid, in a device authentication system. When a list H stored in a personal computer 200 is old, the personal computer 200 acquires a latest version of the list H and a latest version of a list D from outside an internal network, updates the list H stored therein to the latest version, and updates the list D stored therein to the latest version.