PURPOSE: To perform access while being conscious of an operation side and a stand-by side and to effectively utilize an area by separately storing data for which reliability is not important.

CONSTITUTION: The disk area management information of magnetic disk devices 1 and 2 is provided in a disk area management means 4 and an operation area control means 5 switches to the other magnetic disk device and stores the remaining data at the time of the overflow of a storage area of separate operation based on the disk area management information. Also, by assigning the operation area by an instruction from an external input means 6 and registering it to the disk area management information, the access while being conscious of either one of the magnetic disk devices 1 and 2 can be performed.