A storage system has a pool which is made up of a plurality of storage regions supplied by physical disks, and allocates the storage regions within this pool to volumes supplied to host computers, and cancels such allocations. When a volume addition event has occurred, a management computer forecasts, for each volume, the capacity which is to be required at a certain time point in the future, based on usage capacities acquired for each volume, calculates a non-required capacity from this capacity which has been forecast and the capacities of the storage region groups which are already allocated to the volume, decides, based on one or more non-required capacities which have been calculated, whether or not there is a storage region enough to satisfy the capacity requested for a volume which is the object of addition, and, if a positive decision result is obtained, allocates storage regions equivalent to the non-required capacity to the volume which is the object of addition.