To reduce the load on a host for controlling a data doubling. Between a host and two disk subsystems, there is connected a data doubling device. This data doubling device behaves as if it were a unit disk subsystem for the host. The data doubling device doubles a data I/O instruction from the host and issues the doubled data I/O instruction to two disk subsystems. In case either of the disk subsystems becomes faulty, the data doubling device sends the data I/O instruction from the host only to the normal disk subsystem. After the fault recovery, the data doubling device performs a data copy between the disk subsystems.