A storage system comprises a virtual volume configured of multiple virtual areas and a pool configured of multiple actual area groups of different performances. The controller of the storage system manages which actual areas are assigned to which virtual areas and the access load on the actual areas assigned to the virtual areas. The controller, if the access load of a first actual area assigned to a first virtual area is inappropriate for a first actual area group comprising the first actual area, migrates data to a second actual area in a second actual area group which is appropriate for the access load of the first actual area from the first actual area. The controller performs data migration between actual areas for achieving data allocation at a certain point of time based on history information which is the information related to a virtual area at a point of time in the past and an actual area assigned to the virtual area. Data allocation is the allocation of data in a virtual area to an actual area in an actual area group.