A storage system is configured to manage a pool to which multiple virtual volumes (VVOLs) are associated, assign 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 carry out a reassignment process for migrating data inside the RA to an RA of a different tier, based on an access status of the RA. A management system is configured to assume a specified tier has been removed from the available tier pattern of a target VVOL, predict performance of all VVOLs associated with the pool to which the target VVOL is associated, determine whether there is a VVOL for which predicted performance is lower than a required performance, and when such a VVOL does not exist, instruct the storage system to remove the specified tier from the available tier pattern of the target VVOL.