On the OpenFlow network/normal network boundary, the route cannot be appropriately changed under a communication trouble with the controller. In this invention, the controller makes the route formed by internal switches optimal by updating the Flow table of each internal switch. Among internal switches, the internal switch A forms the optimal route with an external switch to become an active internal switch. The internal switch B forms a redundant route with the external switch to become a standby internal switch. When inability of the connection between the controller and the internal switch A is detected, the internal switch A performs a link-down of the port for connecting to the external switch. When the link-down of the port connecting to the internal switch A is detected, the external switch changes the route so that the traffic to the active internal switch A is transferred to the standby switch B.