The present invention is able to improve the processing performance of a storage system by respectively virtualizing the external volumes and enabling the shared use of such external volumes by a plurality of available virtualization storage devices. By virtualizing and incorporating the external volume of an external storage device, a first virtualization storage device is able to provide the volume to a host as though it is an internal volume. When the load of the first virtualization storage device increases, a second virtualization storage device 2 is newly introduced, and connected to the storage system. When a transfer direction is issued from a management terminal, the external volume relating to the selected logical volume is transferred from the first virtualization storage device to the second virtualization storage device.