The correspondence between a plurality of virtual storage positions in a virtual volume for logically holding a snapshot image of a main volume in which data elements transmitted from a higher-level device are written and a plurality of address information elements indicating a plurality of actual storage positions of a plurality of data elements constituting the snapshot image is managed. In the initial backup, all the data elements stored in all the actual storage positions indicated by a plurality of address information elements are backed up, then storage positions where a difference has occurred between the virtual volume and the backup destination storage device is managed, and in the next and subsequent backups, data elements on the storage positions specified from the address information elements corresponding to the differentially managed storage positions are backed up.