A storage system encrypts a plain text received from an external device and stores the cryptogram into a disk unit and, thereafter, decrypts the stored data in the disk unit and transmits the decrypted text to the external device. The storage system includes an encryption unit for encrypting first data received from the external device, a decryption unit for decrypting the encrypted data into second data, and a comparison unit for comparing the first and second data. When the first data and the second data are in disagreement, the first data is encrypted by an encryption unit different from the encryption unit encrypted the first data and the encrypted data is decrypted by the decryption unit into third data, whereupon the first data and the third data are compared. When the first data and the third data are also in disagreement, a failure report is sent to the external device.