PURPOSE: To set a detector at a position separated from a detecting circuit with a coaxial cable and to eliminate a limitation for the position of installation, by using a capacitor which functions as a detector having a capacity change by approach to or contact with a traveling object as a component element of a resonance circuit.

CONSTITUTION: The output of an oscillator 1 is fed to the 1st resonance circuit consisting of a coil 6 and a capacitor 5 as well as to the 2nd resonance circuit comprising a coil 7, a detecting capacitor 3 plus the capacitors 8 and 9 and set in parallel to the 1st resonance circuit. The output of the 2nd resonance circuit is extracted by a coil 12 with the polarities opposite to each other, and the output of the 1st resonance circuit which is extracted by a coil 11 is applied to each parts. Then the output of the sum voltage is fed to a comparator 20 via a wave detecting circuit and a differential amplifier 19. When a traveling object approaches the capacitor 3, and 2nd resonance circuit is put under a detuning state. Thus the balance is lost among the sum voltages, and a signal is delivered from the comparator 20.