When a plurality of applications operating in a host computer or a storage system uses a same pool, actual data constantly allocated to an upper tier drive satisfying the required response performance will continue to increase. Therefore, when data is migrated by IOPS in page units, the access performance such as the response speed may not be satisfied. Therefore, according to the present invention, multiple virtual volumes used in applications are subjected to grouping. Relocation conditions corresponding to performances required for each group are set in advance, and reallocation of tiers according to the relocation conditions is performed.