Abstract: A controller creates a basic volume based on the plurality of physical storage devices, creates a first virtual volume based on the basic volume, creates a first auxiliary volume based on the plurality of physical storage devices, and stores area information for associating a basic storage area with a first virtual storage area. In the case in which a read request to the first virtual storage area is issued, the controller determines whether or not a first auxiliary storage area is associated with the first virtual storage area based on the area information. In the case in which the first auxiliary storage area is not associated with the first virtual storage area, the controller executes a read access to the basic storage area. In the next place, the controller measures a read access frequency to the basic storage area.