When taking a back up of data stored on a disk in a redundant array of inexpensive disks (RAID) device to a plurality of tape devices, writing of the data is first started on all the tape devices. Once writing of the data is completed on the tape device having the highest data writing speed, writing of the data on other tape devices is stopped, and the data is set as clearing target data to be cleared from the disk of the RAID device. The data stored on the tape device, which has the highest data writing speed, is backed up to the other tape devices at a later timing.