PURPOSE: To reduce the number of signal lines and control lines between a control unit, and an operating part and a display part so as to facilitate panel wiring by using the same signal line to transmit a selection signal from the operating part for the operation of a feed water pump, and a signal transmitted from the control unit for the display of the action indicating lamp of the display part.

CONSTITUTION: Between a control unit and an operation display part 24, a display control signal or a water feed pump operating signal can be delivered/ received to/from signal lines LA, LB according to the half-wave current direction of an alternating current AC. In panel transmission, a central processing unit detects operation by the on state of a photocoupler PCa1 or PCb1 turned on by a signal sent through the signal line LA or LB. When there is a water feed pump in operation, a photocoupler PCa2 or PCb2 is turned on to light a corresponding indicating lamp 32A or 32B, and a display control signal is transmitted to an operation display part 24 through the signal line LA or LB.