A plurality of controlling circuits control a plurality of respective shutdown switches to turn ON/OFF. A first signal system transmits, in a direction from one end of the plurality of controlling circuits to the other end of the plurality of controlling circuits, a significant signal during a normal operation and a non-significant signal when the operation needs to be stopped. The second signal system transmits, in a direction from the other end of the plurality of controlling circuits to the one end of the plurality of controlling circuits, a significant signal during the normal operation and a non-significant signal when the operation needs to be stopped. The other end of the plurality of controlling circuits transmits the non-significant signal through second signal system when a signal received through first signal system is the non-significant signal.