At least two magnetic disk units are used as a primary storage unit and a secondary storage unit, and, when an information write request is received, received information is sequentially written into consecutive recording areas in the same recording track of the primary storage unit disregarding write control information included in the received information, and if all recording areas in one track have been exhausted, the writing is continued to recording areas in a recording track adjacent to the one track at the inside or outside thereof, and the received information including the control information is read from the primary storage unit and written into the secondary storage unit based on the control information when the primary storage unit does not perform processing in response to a write or read request.