To provide a computer system in which an encryption-decryption process performed by one encryption-decryption module can be moved to the other encryption-decryption module without stopping the process for a read/write request from a host computer. The computer system has a host computer 200, and a storage system 400, 500 for storing the encrypted data in a storage area. The storage system 400 provides a storage area of the storage system 500 as the storage area of its own for accepting access from the host computer 200. In performing a process for changing for the data encrypted and stored by the destination source 500, the move destination 400 encrypts the data decrypted by the move source 500, the move source 500 further encrypts and stores the data encrypted by the move destination 400 in the move source 500, and after all the data is stored, the move source 500 decrypts and stores the further encrypted data in the block.