A storage system manages a pool to which multiple VVOLs (virtual logical volumes conforming to thin provisioning) are associated, assigns a real area (RA) from any tier in an available tier pattern associated with a write-destination VVOL to a write-destination virtual area (VA), and carries out a reassignment process for migrating data inside this RA to an RA of a different tier than the tier having this RA based on the access status of the RA assigned to the VA. A management system assumes that a specified tier has been removed from the available tier pattern of a target VVOL, predicts the performance of the target VVOL and all the other VVOL associated with the pool to which the target VVOL is associated, determines whether or not there is a VVOL for which the predicted performance is lower than a required performance, and when such a VVOL does not exist, instructs the storage system to remove the specified tier from the available tier pattern of the target VVOL.