The temporary area capacity required to be secured with respect to the whole permanent area is calculated in accordance with the capacity and access frequency of a host computer data permanent area of a disk device contained in the storage system and a disk device of an external storage device that is managed by a storage virtualization function of this storage system. The nonvolatile memory is defined as the temporary area and is used to temporarily store host computer data when a data I/O from the host computer is processed. The required capacity of the temporary area is re-calculated in accordance with an event such as a configuration change in the external storage system.