PURPOSE: To detect the operation states of plural contacts through a couple of connection lines by connecting resistances which have different values to the contacts in series and utilizing variation in the total resistance value depending upon the ON/OFF combination of the contacts.

CONSTITUTION: The resistances 1, 2, and 3 which have the different values are connected to the contacts 4, 5, and 6 in series and a DC power source is connected through a cable 7. A voltage divided by the total resistance and a voltage dividing resistance 10 according to the ON/OFF combination of the contacts 4, 5, and 6 is inputted to an A/D converter 11. The A/D converter 11 outputs a digital signal corresponding to variation in the input voltage from its digital output terminal 9. Thus, the resistances which differ in resistance value are connected to the contacts in series and the variation in the resistance value depending upon the ON/OFF combination of the contacts is utilized to detect the ON/OFF states of the contacts through the couple of connection lines.