The diagram particularly shows a case in which 3.times. transfer is performed by employing two HDDs for storing data+one HDD for storing parity as the objects, and the data and the parity are distributed andstored in a set of three HDDs 30 that are forming the slow-speed-side RAID group.In FIG. 16A, upon a write request, the controller 10 temporarily retains the write data which has been given f