PURPOSE: To effectively utilize the storage area of a disk device capable of executing a simultaneous and parallel operation.

CONSTITUTION: Two host devices 1a and 1b with different disk access methods, the plural disk devices 2a-2n capable of accessing from the two host devices 1a and 1b, an information dividing and connecting means 3 capable of executing the simultaneous and parallel operation of one host device 1a and the plural disk devices 2a-2n and information route change-over means 4a-4n for changing- over the information transfer route of the disk devices 2a-2n in accordance with a request from the respective host devices 1a and 1b are provided. As a result, one host device 1a performs access to the plural disk devices 2a-2n by permitting the plural disk devices 2a-2n to execute the simultaneous and parallel operation. The other host device 1b performs normal access, that is, access to only one disk device.