PURPOSE: To prevent malfunction of an apparatus due to on/off detection originating from poor contact of a switch by judging a failure by a judging means when the number of on/off detection by an on/off detecting circuit to detect on/off's of the switch exceeds a specified value.

CONSTITUTION: When current flows to a transistor(Tr) 46 from a DC power supply terminal VCC through a resistance 47, an L-level signal indicating 'on' of a door switch 28 is given by a detecting circuit 29 to a microcomputer 30. When this switch 28 is turned off, the transistor 46 is put off, and an H-level signal indicating 'off' of the switch 28 is given to the microcomputer 30. When half opening of the door causes bad contact of the switch 28, the voltage impressed on a zenor diode 39 becomes smaller than its inverse voltage, and the transistor 46 is turned off and the circuit 29 senses 'off' of the switch 28. Then the microcomputer 30 controls the load to 'stop', to restore voltage drop at the switch 28, and the circuit 29 senses 'on'. Accordingly the microcomputer 30 detects on/off of the switch 28 in a short time to judge failure.