PURPOSE: To perform good-accuracy detection with simple constitution by passing a glass fiber which is a specimen continuously through a high-frequency electric field generated by a high-frequency transmitter.

CONSTITUTION: A high-frequency chamber 2 of a waveguide type is used to a detector and the glass fiber 3 which is the specimen is continuously passed through this chamber 2. A signal output level is raised over the entire part of the frequency range by generation of a high-frequency discharge if a conductive material such as metal exists in the fiber 3 passing the inside of the chamber 2. Since the output PC0 at the frequency fC apart sufficiently from the oscillation frequency f0, PC/P0 is extremely large. On the other hand, P0 is at the extremely high level and, therefore, SN log(P0/PC) decreases largely. The change in the SN is continuously monitored by a computer 5 using a spectral analyzer 4, by which the conductive material such as metal incorporated in the fiber 3 is detected with the good sensitivity.