A storage system includes a virtual volume, a plurality of RAID groups, a pool unit for managing a plurality of first real storage areas and a controller. If a write command related to the virtual volume is issued from a higher-level device, the controller selects a prescribed second real storage area from among respective second real storage areas included in a prescribed first real storage area, and associates this prescribed second real storage area with a prescribed area inside the virtual volume corresponding to the write command, and which associates one virtual volume with one first real storage area. A migration destination determination unit selects a migration-targeted second real storage area from among the respective second real storage areas associated with the virtual volume, and selects a migration-destination first real storage area, which is to become the migration destination of data stored in the migration-targeted second real storage area.