The media inherent key storing unit 220 prestores an inherent key Ki, the conversion unit 230 generates an encrypted inherent key Ji from the inherent key read from the media inherent key storing unit 220, the random number generating unit 331 generates a random number R1, the encryption unit 252 generates an encrypted random number S1, the decryption unit 333 generates a random number R'1 from the encrypted random number R1, and the mutual authentication control unit 334 compares the random number R'1 with the random number R1 and, if the random number R'1 matches the random number R1, judges that the memory card 200 is an authorized device. If the memory card 200 and the memory card writer have successfully authenticated each other, the memory card writer encrypts a content using a decrypted inherent key. If the memory card 200 and the memory card reader have successfully authenticated each other, the memory card reader decrypts an encrypted content using the decrypted inherent key.