A hard disk A has is constructed to include: a disk 10 in which copy protection information is written in advance in a surface shape such as slits or corrugations in a pre-recording region 13 over a substrate 11 other than a data storage region 12; a pickup unit 20 for reading the copy protection information on the disk 10; and a copy protect unit 42 made operative, when it copies the encrypted data over the data storage region 12 of the disk 10 in response to the demand of the external device B: to perform an authentication with reference to the copy protection information read by the pickup unit 20; and to output the information on the secret key, as contained in the copy protection information, to the external device B which has been recognized to be correct by that authentication.