A technique for realizing a snapshot function is provided, which can reduce data transfer between a server system and a storage subsystem which is necessary during data copy operations between storage devices and reduce the degradation of data access performance of the storage device in operation. In a storage system, a command processed by a CPU of a storage subsystem includes a COPY and WRITE command for performing a data copy process and a data storage process in accordance with a predetermined sequence, and a server system issues the command to the storage subsystem. After receiving the command, the storage subsystem executes a data copy process from a first disk drive to a second disk drive, and subsequently executes a data storage process to the first disk drive, thereby keeping a snapshot of the data stored in the first disk drive.