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.