To exclude any unauthorized device from a system and thereby prevent illegal use of content data, a memory card 10 stores a service key Ksrv for encrypting content key data Kc in a hidden area 14. The memory card 10 stores answer data ACto be obtained when question data Q is fed to a host function F possessed by an authorized host device 20, with encryption applied to the answer data AC by the service key Ksrv. When the question data Q is provided to the host device 20, answer data AH is obtained based on the question data Q and the host function F. A comparison unit 122 judges match or mismatch of the answer data AH and the answer data AC. When they match, the memory card 10 decrypts the content key data Kc encrypted by the service key Ksrv and sends it to the host device 20.