A computer system dynamically assigns the storage capacity from pool volumes to the access target in the higher-level system, and can immediately respond to the change of the status of the pool comprising the pool volumes. A control device sets the access target for the host computer to access, each time a write to the access target occurs from the host computer, from the pool assigned to the access target to the write area of the access target. The control device performs the processing of assigning the storage capacity from the logical volume in the pool. The control device defines the logical volumes in advance, compiles the same into a management group as a preliminary for the pool and, in accordance with the change of the pool status, adds the logical volumes from the management group to the pool.