To surely detect the existence of a metal material, without being influenced by its temperature or surrounding water, etc., simplify the structure of main parts, and improve the operation reliability and life reliability.

A flat plate-like transmitting electrode 7A of a transmitting head 1A and a flat plate-like receiving electrode 7B of a receiving head 1B are opposed at a detecting position SP set on a transport passage of a material 25 transported in an electrically grounded shape, thereby making both electrodes 7A, 7B capacitively coupled. A high frequency voltage signal is applied from an oscillator circuit 16 to the transmitting electrode 7A. A signal processor circuit 17 generates a detection signal when the voltage level developed on the receiving electrode 7B is below a predetermined reference threshold.