In the present invention, a controller completes flow entry settings for a switch before communication of a virtual machine commences. Specifically, the controller sets information of the virtual machine and information of a server which operates the virtual machine. The controller detects, by way of the switch, the information of the server which is controlled by the switch. When the set information of the server and the detected information of the server match, the controller sets in the switch a flow entry with regard to packets having said virtual machine as the destination, and this setting is performed on the basis of the information of the virtual machine on the server. The virtual machine then commences communication.