PURPOSE: To improve the using efficiency of a disk buffer and an I/O speed by using plural disks in common with plural disk buffers.

CONSTITUTION: If no necessary data exist when a control device 1 detects all blocks in the disk buffer 5, blocks in the disk buffer 5 of which data valid/ invalid flags are invalidated are successively retrieved. If a data invalid block exists, necessary data in a disk 7 are read out, written in the block and transferred to a main storage device 3, the drive number of the block is updated, the data valid/invalid flag is validated, and the access order is updated. If there is no data invalid block in the disk buffer 5, the oldest data in the disk buffer 5 are retrieved, read out and transferred to the main storage device 3.