A storage control technique with which, even if not data, but only its management information is deleted, a host system can recognize storage areas storing unnecessary data and so use them effectively. When a file system in the host system deletes the management information for data stored in a page in a pool volume, a control circuit writes ‘0’ in all the areas in the deletion target page in response to a command from the host system. The control circuit then detects the page where ‘0’ is written in all of its areas and releases it from allocation to the corresponding virtual volume.