In a structure where first and second storage devices structure a mirrored pair, at a first time, an updating process to the second storage device is stopped, and management using first and second update-management tables is started. At a second time on or after the first time, management of the first update-management table is started from a non-updated state, and data of the second storage device, stored in the data blocks indicated by the second update-management table as being updated, is copied to a backup medium. At a third time after the second time, data of the first storage device, stored in the data blocks indicated by the second update-management table as being updated, is copied to data blocks of the second storage device corresponding to the data blocks of the first storage device.