A storage system coupled to a host has a nonvolatile semiconductor storage device that includes a nonvolatile semiconductor memory configured by a plurality of pages, and a storage controller coupled to the semiconductor storage device. In the case where data stored in the plurality of pages become unnecessary, with this plurality of pages being the basis of a region of a logical volume based on the nonvolatile semiconductor storage device, the storage controller transmits, to the nonvolatile semiconductor storage device, an unnecessary reduction request for reducing the number of pages that are the basis of the region having the unnecessary data stored therein. On the basis of the unnecessary reduction request, the nonvolatile semiconductor storage device invalidates the plurality of pages that are the basis of the region having the unnecessary data stored therein.