To obtain a stable detection result while obtaining a good detection sensitivity in a simple constitution.

Exciting coils 13c, 13d are disposed separately from a detecting coil 12 for detecting the displacement, based on the balance between the pair of exciting coils 13c, 13d. This cancels impedances ascribed to d-c resistances thereof, etc., and a compact core 11 is used to obtain a change quantity remaining after this cancel at a high sensitivity with a satisfactory linearity. Thus a low-cost circuit allows the stable detecting operation, irrespective of the ambient temperature change, without using any constant-current circuit as in the prior art.