In the printed circuit board 120, the HDC circuit 121 is connected to the CPU 150 through an interface such as not illustrated small computer system interface (SCSI) bus or the like so as to send/receive various commands (read command, write command and the like), write data to be written into the magnetic disks 111 1-111 n read data read out from the magnetic disks 111 1-111 n and the like.