A playback instructing unit in a disk playback controller has a function to instruct a CPU to play back an optical disk at different access positions in an absolute authentication process and an arbitrary authentication process. An absolute decision unit has a function to be activated by an absolute authentication instruction from the playback instructing unit and authenticate the decided result from an authentication decision means according to a first rule (which declares normal authentication based on a normal decision). An arbitrary decision unit has a function to be activated by an arbitrary authentication instruction from the playback instructing unit and authenticate the decided result from the authentication decision unit according to a second rule (which declares normal authentication based on an abnormal decision). Even if a special device which can normally authenticate an optical disk that should not be authenticated normally is connected to a disk access mechanism, the optical disk that should not be authenticated normally or the special device can reliably be detected to forcibly interrupt any process being carried out, so that the optical disk can effectively be prevented from being illegally copied.