Each of the storage devices 1, 2 is provided with transmission queues 306a, 306b with respect to each of transmission paths A and B. The storage device 1 stores identical request data in each of the transmission queues 306a and 306b, and transmits each data to the counterpart storage device 2, respectively via the transmission path A and B (S811, 812). The storage device 2, being a counterpart, stores the request data or response data arrived earlier in the receive history information table 300 (S817), and discards the request data arrived later (S903, 904). Furthermore, if the storage device 1, being a transmission source, receives at least one response data against the request data, the storage device 1 searches the transmission queues 306a, 306b, and if there remains request data in any of the transmission queue, deletes the request data (S909).