Improvement of read/write access performance with respect to a disk is proposed. A controller manages a first volume format LDEV, in which each distributed user data area and each distributed information area among a plurality of the distributed user data areas for storing a data part and a plurality of the distributed control information area for storing a control information part, are targets that capacity is changed. The controller also manages a second format LDEV, which include a plurality of groups each of which is formed from one distributed user data area and one distributed control information area, and in which each group is a unit that capacity is expanded in a real storage area. The controller converts a data address of the data part belonging to the first volume format LDEV into a data address of a data part of the second volume format LDEV in order to execute input/output processing with respect to the data part, when received an request for access to the data part belonging to the first volume format LDEV.