PURPOSE: To obtain high detecting sensitivity stably by controlling AC which is flowed through a displacement converting capacitor at a constant value, and providing a system which is different from a feedback control system that holds a good linear proportional relationship.

CONSTITUTION: A voltage vp is applied to the displacement conversion capacitor 12 from an oscillating coil 11b of an oscillator 11. A voltage vd across current detecting resistor 13 is rectified 14, and a DC voltage Vd is obtained. The voltage Vd is compared 16 with a reference voltage 15. The resulting output voltage Vs is supplied to the oscillator 11 as a power source. A DC voltage VL is obtained by rectifying 21 an AC voltage vb that is generated in an output coil 11c of the oscillator 11. The DC voltage VL is converted 17 into a DC I. The DC I is supplied to a circuit 14, a comparator 16, and a circuit 17 through a constant current circuit 18. In this constitution, since the power source voltage Vs of the oscillator 11 is proportional to the output AC voltage vp of the oscillator, the displacement can be detected.