An authentication processing apparatus includes an authentication unit, having a circuit that performs authentication phases included in processing for authenticating an external device. A command holding unit holds a first command that indicates whether or not each of the authentication phases is performed by the authentication unit. An authentication controller causes the authentication unit to perform an authentication phase that is indicated, by the first command, to be performed by the authentication unit. A CPU performs software processing of an authentication phase that is indicated, by the first command, not to be performed by the authentication unit.