A virtual parity group including virtual storage drives is formed. A redundant data set is stored in a virtual stripe array including virtual stripes that are virtual storage regions of a prescribed size of the respective virtual storage drives constituting the virtual parity group. The virtual stripes of the virtual stripe array are mapped to different physical storage drives, respectively. Virtual chunks including one or more virtual stripe arrays formed in the virtual parity group. Physical parity groups including the physical storage drives are formed. The virtual stripes of the virtual chunks in a distributed manner are mapped to the physical parity groups. One or more virtual chunks are selected from among the virtual chunks. Storage regions corresponding to the selected one or more virtual chunks are converted to spare regions in the physical storage region.