A host computer and primary and secondary storage devices are connected as capable of communicating with each other via a network, the primary and secondary storage devices functioning as data storage means of the host computer. The host computer transmits data to be written and a write request Wa to the primary storage device in writing of data into the primary storage device and transmits the data to be written and a write request Wb to the secondary storage device at any point in time after the foregoing transmission. The primary storage device receives the write request Wa and stores the data to be written. The secondary storage device receives the write request Wb and stores the data to be written.