1. A storage virtualization switch, comprising: a plurality of physical ports, to which one or more host computers and one or more physical storage units can be connected; storage virtualization means for constituting one or more virtual storage units by using a part or a whole of a storage area of a physical storage unit; virtual target constituting means for constituting one or more virtual targets, which acts as a target of a virtual storage unit with respect to a host computer; first control means for controlling said virtual target constituting means to generate a virtual target; second control means for controlling said virtual target constituting means to selectively put the virtual target into an effective state, in which the virtual target is allowed to be connected to the host computer, or an ineffective state, in which the virtual target is prohibited to be connected to the host computer; and third control means for controlling said virtual target constituting means to generate one or more dummy virtual targets, which corresponds to the one or more virtual targets, and for controlling said virtual target constituting means to set a World Wide Name (WWN), which is not recognized by the host computer, as a WWN of a dummy virtual target, wherein said virtual target constituting means assigns a port ID number to the virtual target when the virtual target is put into the effective state, said virtual target constituting means releases the port ID number of the virtual target when the virtual target is put into the ineffective state, said second control means puts the dummy virtual target, which corresponds to the virtual target, into the effective state after the virtual target is put into the ineffective state when the virtual target is put into the ineffective state, and said second control means puts the virtual target into the effective state after the dummy virtual target, which corresponds to the virtual target, is put into the ineffective state when the virtual target is put into the effective state.