In a storage controller which stores data provided from a host system, creates a snapshot, which is a data image of a production volume at a given point in time, at regular or irregular intervals, and holds difference data of the snapshot in a pool volume, multiple pool volumes are configured and a different pool volume is assigned to each copy pair including a group of production volumes or a single production volume that does not belong to any group of volumes so that difference data of each copy pair can be saved in its associated pool volume.