A storage system comprises multiple virtual volumes (VVOLs) and multiple pools. Each pool is configured of multiple actual area groups of different performances. The storage system accesses actual areas assigned to virtual areas identified by access commands from an access source. A controller manages pool condition information which is the information showing which actual area is assigned to which virtual area and the access load related to the virtual areas. The management system of the storage system comprises management information showing a performance requirement and a performance for each VVOL. The performance of a VVOL is the performance identified with reference to the pool condition information. The management system, with reference to the management information, identifies a VVOL whose condition is inappropriate and, for changing the condition of the identified VVOL from inappropriate to appropriate, performs migration procedure related to the pool made to correspond to the above-mentioned identified VVOL.