A pool is formed based on a plurality of storage devices. This pool is constituted by a plurality of real pages. Real pages of different lengths are included in this plurality of real pages. Among a plurality of virtual pages which make up a virtual volume, a controller compresses a write data element for a write destination virtual page, selects a real page of a real page length based on the data length of a data unit including the compressed write data element, and allocates the selected real page to the write destination virtual page.