PURPOSE: To eliminate the influence exercised by a distributed capacity, by a method wherein a capacity type sensor is used as a feedback capacity of an inversion amplifier to form a Miller integrator.

CONSTITUTION: A capacity C1, in which an electrostatic capacity changes in proportion to a displacement of a process amount, is connected as a feedback capacity between an input and an output of an inversion amplifier Q1, and a resistor R1 is inserted in series into the input of the inversion amplifier Q1 to form the Miller integrator. Simultaneously, a trigger circuit Q2, having a hysteresis property, is inserted in series between the Miller integrator and the resistor R1 to form as a whole a relaxation oscillator which charges and discharges for the duration of a period corresponding to values of the resistor R1 and an electrostatic capacity C1. If the gain of the inversion amplifier Q1 is extremely large, the input potential of the inversion amplifier Q1 is kept constant through the integral action, whereby charging and discharging toward a distributed capacity CS1 are not conducted, and the presence of the distributed capacity CS1 can be ignored. Additionally, of the output impedance of the inversion amplifier Q1 is extremely small, the presence of a distributed capacity CS2 can also be ignored.