In a computer system having a storage system in which storage units are hierarchically configured, a management method for accurately grasping the capacity available to the computer is disclosed. In a computer system in which a management computer manages the capacities of storage units for storing data used by the computer, the management method is typically realized by a storage management system comprising a group of first-level storage units each containing volumes for storing data used by the computer, a group of second-level storage units each of which is hierarchically linked to, and physically connected through a communication path to, one of the first-level storage units and contains volumes for storing data used by the computer, a means for collecting volume information from the first-level and the second-level storage units, a means for collecting inter-volume hierarchy information, and a means for calculating the total effective capacity to the computer based on the volume information and the inter-volume information thus collected.