In order to prevent the degradation of performance of a storage apparatus caused by dynamic reallocation, the storage apparatus performs reassigning to a logical page the first physical page which is the physical page provided by the physical drive in Tier 1 which is the higher hierarchy than Tier 2 which is the hierarchy of the physical drive which provides the second physical page which is the physical page currently assigned to the logical page and, at the same time, by making the contents of the second physical page identical to the contents of the first physical page, performs the first migration for the logical page, associating and managing the second physical page and the first physical page and, when performing the second migration by reassigning the physical page provided by the physical drive in Tier 2 to the logical page to which the first physical page is assigned, and performs the second migration by reassigning the relevant second physical page to the logical page again when the second physical page is associated with the first physical page.