To prevent erroneous discrimination of an input signal caused by transient voltage drop of a main power source.

A first reference voltage generation circuit 23 which divides a battery voltage (+B) by two voltage dividing resistances R1 and R2 to generate a first reference voltage Vth1 and a second reference voltage generation circuit 24 which divides an output voltage Vcc of a regulator 16 by voltage dividing resistances R3 and R4 to generate a second reference voltage Vth2 are provided. Lower one of these first and second reference voltages Vth1 and Vth2 is selected as a reference voltage Vth by a selection circuit 18, and this reference voltage Vth is inputted from an output terminal 29 of the selection circuit 18 to an inverted input terminal (-) of a comparator 19. Meanwhile, a voltage Vin of the switch signal inputted from a signal input terminal 15 is inputted to a non-inverted input terminal (+) of the comparator 19 and is compared with the reference voltage Vth; and in the case of Vin≥Vth, the output of the comparator 19 goes to the high level to judge that the switch 11 is turned on.