In a case where the decrypted data M, for which copyrights are claimed, is stored in a medium such as a digital versatile disk (DVD) RAM or a hard disk, or in a case where it is transferred externally via a network, the decrypted data M is mandatorily re-encrypted by the device driver 71, i.e., the disk driver 67 and the network driver 68, using the second changeable key K2: