A first storage system is connected to a second storage system, and an external device within the first storage system is provided to a host as a device of the second storage system. The second storage system includes a cache control section having a cache adaptors, each controlling a disk and a cache, a protocol conversion section including protocol adaptors that switch requests from the host to appropriate ones of the cache adaptors, a management adaptor, and an internal network that mutually connects the cache adaptors, the protocol adaptors and the management adaptor. The first storage system being connected to any of the protocol adaptors is connected to the second storage system. The second storage system executes a processing for the external device by the cache control section, or connects to the first storage system through the protocol conversion section without the cache control section executing processing for the external device.