PURPOSE: To prevent the generation of spike noise, by providing insulator guards in a gap between electrodes facing to each other through which objects are transferred.

CONSTITUTION: The upper face of an inner shield portion 3 of a ground electrode 4 and a running portion on an active electrode 6 are covered with resin guards 100a and 100b respectively. A ceramic resonator 22 outputs voltages corresponding to reference frequency signals inputted from an OSC 21, and outputs signals amplitude modulated corresponding to variations of the electrostatic capacitance between electrodes 4 and 6. A detector 23 outputs voltages V out obtained by envelope detection of the amplitude modulated signals. The variation of thickness or material of the objects can be known from the voltages V out. By providing resin guards 100a, 100b, objects cannot be contacted to a metallic case, hence abrupt disturbances in high frequency currents can be eliminated to prevent the generation of spike-shaped noise.