A storage system including a storage apparatus, a host computer and a management server for managing said storage apparatus and said host computer, the storage apparatus providing a dynamically extensible dynamic logical volume, and having a plurality of physical devices to configure a plurality of storage extents, the host computer having a file system for inputting and outputting data to and from said dynamic logical volume, said host computer comprising a capacity utilization acquisition unit for acquiring the capacity utilization of said file system; wherein said storage apparatus comprises: an allocation unit for allocating, when said dynamic logical volume receives a write request from said host computer and a storage extent of said storage extents corresponding to said write request is not allocated to said dynamic logical volume in predetermined units, a storage extent of said storage extents to said dynamic logical volume; a management unit for managing said storage extent recognized as being currently used by said file system among said storage extents allocated to said dynamic logical volume by said allocation unit; a storage extent quantity acquisition unit for acquiring the number of said storage extents allocated to said dynamic logical volume corresponding to said file system, and for returning the acquired number to the host computer in response to a request from the host computer therefor; and a release unit, responsive to the host computer, configured to release the allocation of a storage extent of said storage extents that is not being used by said file system from said dynamic logical volume; wherein in response to a request from the management server, the host computer returns to the management server the capacity utilization of said file system acquired by the capacity utilization acquisition unit, and the number of said storage extents allocated to said dynamic logical volume corresponding to said file system acquired by the storage extent quantity acquisition unit; wherein said management server comprises a release decision unit: for deciding said dynamic logical volume to release said storage extent based on the received capacity utilization and the received number of said storage extents; and for deciding said dynamic logical volume to release said storage extent to be said dynamic logical volume having a divergence between the received capacity utilization and the received number of said storage extents which exceeds a prescribed threshold; and wherein said management server is configured to issue a release processing command to the host computer when it is decided to release said storage extent; as a consequence of receiving the release processing command, the host computer is configured to issue an extent release processing command to the storage apparatus; in response to the storage apparatus receiving the release processing command, the release unit is configured to release the allocation of said storage extent to said dynamic logical volume. an allocation unit for allocating, when said dynamic logical volume receives a write request from said host computer and a storage extent of said storage extents corresponding to said write request is not allocated to said dynamic logical volume in predetermined units, a storage extent of said storage extents to said dynamic logical volume; a management unit for managing said storage extent recognized as being currently used by said file system among said storage extents allocated to said dynamic logical volume by said allocation unit; a storage extent quantity acquisition unit for acquiring the number of said storage extents allocated to said dynamic logical volume corresponding to said file system, and for returning the acquired number to the host computer in response to a request from the host computer therefor; and a release unit, responsive to the host computer, configured to release the allocation of a storage extent of said storage extents that is not being used by said file system from said dynamic logical volume; wherein in response to a request from the management server, the host computer returns to the management server the capacity utilization of said file system acquired by the capacity utilization acquisition unit, and the number of said storage extents allocated to said dynamic logical volume corresponding to said file system acquired by the storage extent quantity acquisition unit; wherein said management server comprises a release decision unit: for deciding said dynamic logical volume to release said storage extent based on the received capacity utilization and the received number of said storage extents; and for deciding said dynamic logical volume to release said storage extent to be said dynamic logical volume having a divergence between the received capacity utilization and the received number of said storage extents which exceeds a prescribed threshold; and wherein said management server is configured to issue a release processing command to the host computer when it is decided to release said storage extent; as a consequence of receiving the release processing command, the host computer is configured to issue an extent release processing command to the storage apparatus; in response to the storage apparatus receiving the release processing command, the release unit is configured to release the allocation of said storage extent to said dynamic logical volume.