1. A management method for managing a storage apparatus, wherein the storage apparatus manages, as different types of storage tiers, each of storage areas provided by multiple types of storage devices with different levels of performance, constitutes a pool from the different types of storage tiers, provides a virtual volume to a host system, dynamically allocates a storage area of specified size on a page basis from the pool to the virtual volume in response to a write request from the host system to the virtual volume, and regularly or irregularly switches a storage tier, from which a page is allocated, to another type of storage tier, depending on the number of accesses per unit time to the page allocated to the virtual volume, and wherein the management method comprises: a first step of detecting, as a high-load storage tier, a storage tier whose number of accesses per unit time has reached a performance threshold and whose used capacity has not reached a capacity threshold; a second step of detecting a virtual volume to which a page is allocated from a pool constituted form the high-load storage tier, selecting a virtual volume that is a migration candidate whose data is to be migrated, from among the detected virtual volume, and selecting, as a migration source pool, a pool from which a page is allocated to the virtual volume; a third step of selecting, as a migration destination pool, a pool whose at least one constituting storage tier has the number of accesses per unit time not reaching the performance threshold, and which is different from the migration source pool; a fourth step of deciding a migration candidate pair by combining the selected migration-candidate virtual volume and the selected migration destination pool; a fifth step of calculating, for each the migration candidate pair, response performance of an entire system before and after migration when data written to the migration-candidate virtual volume is to be migrated to the migration destination pool; a sixth step of deciding a migration candidate pair which is a migration object, based on the response performance of the entire system before and after the migration; and a seventh step of controlling the storage apparatus so that data of the migration-candidate virtual volume constituting the decided migration candidate pair will be migrated to the migration destination pool constituting the migration candidate pair.