Provided are a computer system and its data control method that enable safe backup of data and reduction in the capacity of data to be backed up. A control processor refers to a differential and, if a differential bit is “0,” encrypts data of an online VOL with an encryption circuit by using key information, and transfers the encrypted data to a second storage system. If the differential bit is “1,” the control processor performs redundancy elimination processing to journal data, thereafter compresses the journal data with a compression circuit, encrypts the compressed journal data with an encryption circuit according to the key information, and transfers the journal data that was encrypted after compression to the second storage system.