A DVD terminal (20) makes a request for sending a sub-content to an authentication server (40) (in this case, attaches the "model name and serial number" of the DVD terminal (20)) (S302). The authentication server (40) generates a random number R (S304) and sends it to the DVD terminal (20) (S306). The DVD terminal (20) reads a terminal key and a terminal ID that are stored in a terminal information memory unit (25), decrypts the received random number R using a terminal key (SK_X) (S308) and sends this and the terminal ID (ID_X) to the authentication server (40) (S310). The authentication server (40) verifies the random number R and the terminal ID that are received from the DVD terminal (20) and encrypted, and judges whether the DVD terminal (20) is the authenticated terminal or not (S312).