The amount of data to be stored in a semiconductor nonvolatile memory can be reduced and overhead associated with data processing can be reduced. When a microprocessor 112 receives a write request from a host computer 300 and data D1 to D3 exist in a cache slot 117, the microprocessor 112 reads the LBA of each piece of the data, manages each piece of the data D1 to D3 using a bitmap table 118 by associating them with their LBAs, generates a specific command CMD based on the LBAs of the data D1 to D3, adds the data D1 to D3 and addresses ADD1 to ADD3 indicating where the data D1 to D3 are to be stored, to the specific command CMD, and sends it to an FMPK 30. The FMPK 130 stores each piece of update data in a specified block in the flash memory 135 based on the specific command CMD.