Abstract: A circular array of equally spaced, identical stationary electrodes are arranged about the center of the array and a rotary electrode is substantially arranged on a surface of the rotary member to be monitored in such a manner that M among N stationary electrodes have a maximum capacitance by being in confronting relationship to the rotary electrode according to the angular position of the rotary member, a maximum capacitance is in proportion to .theta. which is the azimuthal angle of the radius as measured from the initial end of the stationary electrode. The angular position of rotary member is identified from the stationary electrodes which are in confronting relationship to rotary electrode and are identified by measuring the output signal owing to a maximum capacitance while scanning the stationary electrodes.