PURPOSE: To improve a linearity and to detect with high sensibility and high accuracy an opposite distance change by detecting via a load resistance the output voltage at the prescribed point with a balancing capacitor having the prescribed capacity in series with a twin plane sheet capacitor changing the opposite distance driven by an alternating current.

CONSTITUTION: A twin plane sheet capacitor 1 which the prescribed capacity is changed with Δc according to the change Δd of opposite distance and a balance capacitor 6 having the same capacity with the prescribed capacity are connected in series and driven by alternating current via a transformer 9. The mid-point of the secondary circuit of the transformer 9 and a voltage E0 corresponding to the capacity change Δc between the intermediate connecting points of a capacitor 1, 6 are detected by a load resistance 7 connected to these points. By this constitution the characteristics of the change Δd and the change Δc and the characteristics of the change Δd and the voltage E0 are mutually compensated and the detected voltage is lineally changed for an opposite distance and the opposite distance change is detected with high sensibility and high accuracy.