A disk control device stores write requests from a cache memory or reads commands from a host in a queue for a disk drive in chronological order. When the number of write requests stored in the queue for the disk drive is greater than a predetermined value, the storage location of write requests is changed to a queue for an extra disk drive, and the write requests are stored in the queue for the extra disk drive. When the number of write requests stored in the queue for the disk drive becomes smaller than a predetermined threshold, the write requests stored in the extra disk drive are written back to the disk drive.