After the DVD player 1 cooperates with the personal computer 2 to perform an authentication procedure in such a manner as described above, it requests, in step S21, the magneto-optical disk apparatus 3 for an ID. When the ID requesting signal is received via the 1394 interface 36 in step S22 by the magneto-optical disk apparatus 3, firmware 30 (FIG. 10) in the magneto-optical disk apparatus 3 read