1. A memory management apparatus for accessing a physical page mapped according to mapping information to a logical page that includes a logical address specified by an access request, the mapping information showing a one-to-one mapping between a plurality of logical pages and a plurality of physical pages, the logical pages being defined by dividing a logical address space by a predetermined size, each of the physical pages functioning to physically retain data of the predetermined size and degrading in storage performance each time an access is made thereto, the apparatus comprising: an access frequency index storage unit operable to store an access frequency index for each logical page, the access frequency index indicating an occurrence frequency of an access request specifying a logical address included in a corresponding logical page; a degradation index storage unit operable to store a degradation index for each physical page, the degradation index indicating a degree of degradation in storage performance of a corresponding physical page; and a degradation leveling unit operable to (i) exchange retained data between a first physical page and a second physical page, the first physical page being mapped according to the mapping information to a specific logical page of which an access frequency index is greater than or equal to a first threshold, and the second physical page having a degradation index that is less than or equal to a second threshold, and (ii) update the mapping information so as to show that the specific logical page is mapped to the second physical page.