When the capacity of a buffer region runs short, storing a first generation data set, a second generation data set, and a third generation data set, a control unit selects the first generation data set as a designated data set. The control unit creates, and stores in a storage unit, update record information based on the first generation data set. The control unit identifies the position where the first generation data set is to be stored in the copy source volume according to correspondence relation information, and updates the copy source volume with the first generation data set. The control unit deletes the first generation data set from the buffer region, and also deletes the correspondence relation information corresponding to the first generation data set from the storage unit.