Resources of a storage apparatus are utilized effectively by increasing and reducing a capacity of a differential LU used in a snapshot. In a disk array apparatus including a control processor which controls reading and writing of data with respect to a first logical volume which is generated using storage areas of a plural disk drives, performs control such that data in the past stored in the first logical volume is written in a second logical volume as differential data for each generation, and manages the differential data, the control processor manages a pool management table, in which a logical volume usable as the second logical volume is registered, and a pool addition object management table, in which a logical volume which can be added to the second logical volume is registered, and moves the logical volume from the pool addition object management table to the pool management table to thereby increase a capacity of the second logical volume.