A computer system comprises a primary volume 22P having a plurality of storage blocks P, and a differential volume 22D having a plurality of storage blocks D. Differential data corresponding to one among the plurality of storage blocks P is stored in at least one among the plurality of storage blocks D. A control device 210 identifies differential data which is unnecessary for a host device 100, based on a snapshot management table S or C indicating whether data has been read by the host device 100 from any one of storage blocks P, and on a differential management table 400 indicating to which storage block P differential data corresponds and in which storage block D the data is stored, and performs at least one among removal of the identified differential data from the differential volume 22D, and removal of an unnecessary information element from the differential management table 400.