A storage system has a storage pool for at least one of a logical volume having a plurality of slices, a memory for storing a slice table indicating whether each slices of the logical volume is assigned from the storage pool or not, and a controller for receiving a release command for releasing one of the slices, sending out a response indicating the releasing of one of the slices corresponding to the release command has completed regardless of an execution of processing corresponding to the release command, duplicating a copy target area included in the one of the slices corresponding to the release command into a copy target volume included in the storage pool, and releasing the one of the slices upon completely duplicating the copy target area into the copy target volume.