Proposes the realization of the performance stability operation of virtual volumes and the stabilized access performance for the virtual volumes in the storage apparatus. In the storage apparatus which configures a pool of multiple types of storage tiers of different performances, provides virtual volumes to the host computer and, in accordance with a write request from an application to a virtual volume, assigns pages to the relevant virtual volume from the pool, a policy associating an application with the virtual volume with which the relevant application reads/writes data, a storage tier to assign pages to the relevant virtual volume, and the priority of the relevant storage tier is managed and, in accordance with a write request from the application, complying with the corresponding policy, among the storage tiers associated with the corresponding virtual volume, pages are assigned from the storage tier of the higher priority to the relevant virtual volume.