PURPOSE: To prevent erroneous functions and troubles of a machine by detecting different currents corresponding to the reactances of a capacitor and a coil and discriminating AC frequencies based on the detected currents.

CONSTITUTION: In a circuit receiving a plurality of AC current having different frequencies, a coil 2 having a predetermined inductance is connected in series or parallel with a capacitor 1 having predetermined capacitance, and a detecting means 3 detects different currents corresponding to the reactances of the capacitor 1 and the coil 2. By such arrangement, AC frequencies can be discriminated based on the current detected through the detecting means 3. Consequently, erroneous connection of power source can be detected automatically resulting in prevention of erroneous functions or troubles of a machine.