PURPOSE: To maintain the condition of a switch control relay circuit even when a logic power source has short-circuited by connecting a resistor for power stoppage between one end of the relay and an AC/DC converting circuit, and connecting the contact of the relay in parallel with the resistor for prevention of power stoppage.

CONSTITUTION: A relay coil 19a is connected between the input terminals of a logic power source 9, and a resistor for power stoppage prevention is connected between one end of this relay coil 19a and an AC/DC converting circuit 8. A relay contact 19b is connected in parallel with this resistor 18 for prevention of power stoppage. The resistance value of the resistor 18 for prevention of power stoppage is set so that the voltage that the resistance values bears when the logic power source 9 shorts and the relay contact 11b is broken may be the voltage which can preserve the condition of a switch control relay circuit 7. Hereby, the relay contact 19b is broken at shortage of the logic power source, and the condition of the switch control relay circuit 7 can be maintained by the voltage across the resistor 18 for prevention of power stoppage, and there is no fear of a switch 4 being cut.