PURPOSE: To prevent the malfunction by arranging the constitution such that excessive AC voltage is not applied to a signal input circuit even if a signal input terminal is erroneously connected to one side of power source terminal.

CONSTITUTION: The power terminal 2 of an AC power source 1 is connected to the power receiving part 8 of a signal input circuit 7 through a power circuit 4, and the power terminal 3 is connected to the power common part 9 of the signal input circuit 7, respectively. A diode 14 as a directional element is connected between the signal input part 10 and the signal input terminal 13 of the signal input circuit 7. In the case of inputting a signal into the signal input terminal 13 so as to drive the signal input circuit 7, a diode 15 as another directional element and a resistor 16 are connected between the power terminal 3 and the diode 14. Hereby, when the signal input terminal 13 is connected to the power terminal 2 by mistake, the resistor 16 is burned off by the current from the power terminal and the signal input terminal 7 is protected.