PURPOSE: To eliminate the influence of the leakage impedance intervening in the output side of a sensor by applying negative feedback between the output side of an output part and the input side of an AC conversion part.

CONSTITUTION: The excitation signal emitted by a signal source SG is applied to a differential impedance type sensor DS, and the mechanical displacement detection output generated by the sensor DS is converted to DC by an AC to DC conversion part AC/DC. This output is applied to an output part OT. Part of the output in the output part OT is applied to a DC to AC conversion part DC/AC via a potentiometer RV for feedback and an integrating circuit by a resistor R1 and a capacitor C5. A chopper is constituted by switches S3, S4 in the part DC/AC, and the output applied from the part OT is converted by this chopper to an AC signal which is applied to the input side of the part AC/DC. Hence, the potential fluctuation of the movable electrode MP of the sensor DS is supressed by the negative feedback effect from the part OT, and therefore the influence of the leakage impedance intervened in the output side of the sensor DS is suppressed.