Conventionally, when a switch virtualizing a storage (storage virtualization switch) is installed in a computer system including an SAN, a host computer, and a storage device, since a port ID of a virtual storage and a port ID of a storage device assigned to the virtual storage are different, the computer system has to be suspended at the time of installation of the storage virtualization switch. According to one embodiment of the invention, the storage virtualization switch installed in the computer system assigns a port ID to a port of a virtual storage generated by the storage virtualization switch so as to be equivalent to a port ID of an existing storage device and, in the case in which the port ID is designated as an access destination by an access request from one computer to the storage device, sends the access request to the virtual storage.