1. A storage system comprising: a plurality of physical storage devices which become a basis of a pool as a physical storage area; and a storage control apparatus that is coupled to the plurality of physical storage devices, and is configured to provide a virtual volume as a virtual logical volume to a host computer, to allocate a physical area from the pool to a virtual area to which a write destination of the virtual volume belongs, and to write data of a write target to the allocated physical area, wherein the storage control apparatus performs, for each virtual area to which a physical area is allocated, one of coarse-granularity management for managing a correspondence relationship between a virtual area and a physical area in a first size unit, and fine-granularity management for managing a correspondence relationship between a virtual area and a physical area in a second size unit smaller than the first size unit, the storage control apparatus is configured to manage mapping information that expresses a correspondence relationship between a virtual area and a physical area, the storage control apparatus is configured to perform at least one of change of fine-granularity virtual area to coarse-granularity virtual area, and change of coarse-granularity virtual area to fine-granularity virtual area, based on the number of duplication areas of each virtual area, the duplication area of the virtual area is an area where data that are present in a physical area allocated to the virtual area and are duplicated are stored, the coarse-granularity virtual area is a virtual area for which the coarse-granularity management is performed, and the fine-granularity area is a virtual area for which the fine-granularity management is performed.