1. A storage virtualization switch, comprising: a plurality of physical ports, to which at least one host computer and at least one physical storage unit can be connected; storage virtualization means for constituting at least one virtual storage unit by using a part or a whole of a storage area of the physical storage unit; virtual target constituting means for constituting at least one virtual target, which acts as a target of the virtual storage unit with respect to the host computer; first control means for controlling said virtual target constituting means to generate the 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 at least one dummy virtual target, which corresponds to the 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.