After an initialization process (S10), the access controller of the external storage device performs authentication using a password (S20, S30). If authentication is successful, the successfully authenticated state is saved, and the host computer is notified that the HD storage portion is accessible (S40). Subsequently, it is decided whether SOF packets are being periodically sent from the host computer (S50), and if periodic sending has been interrupted (S50: NO), the connection to the host computer will be deemed lost, and the saved authenticated state will be deleted (S60).