PURPOSE: To efficiently use the capacity to achieve a duplexed file by discriminating a duplexed area and a non-duplexed area of plural bar code disks and switching the access method by the request from a processor.

CONSTITUTION: A sector number discriminating part 9 of a disk controller 7 stores the start sector and the end sector of a part to be made duplexed and discriminates whether the sector number given at the time of read or write belongs to the duplexed part or not, and the dual write/read control of stored data to hard disk units 1 and 3 is performed in accordance with the storage write command and stored data or the storage read command from a processor 5. Thus, a part of hard disk units 1 and 3 is made dual, and only data which should be essentially made duplexed is stored there, and thereby, the capacity is efficiently used to achieve the duplexed file.