PURPOSE: To make the completion of windowing in Si-oxide films possible without damaging semiconductor substrates by detecting the change in the generation intensity of CO generated in a reaction chamber during etching, converting the same to an electrical signal and turning off the applied high frequency voltage.

CONSTITUTION: A high frequency voltage is applied between an upper electrode 4 and a lower electrode 3 in a reaction chamber 1 by a high frequency oscillator 5 to generate reacting gas plasma. The gas plasma etches the Si-oxide films of samples 10 to be etched and forms CO during the etching reaction. Since CO generates intrinsic light emission, the emission intensity thereof is detected with an optical detector 11, and is converted and amplified to an electric signal by an amplifier 12. The electric signal is fed to a high frequency oscillator 5. When the windowing of the Si oxide films on the samples 10 is completed, the generation of CO is decreased sharply. The change thereof is detected with the detector 11 and is transmitted to the amplifier 12. The oscillator 5 is stopped by the electric signal fed from the amplifier 12 and the irradiation of plasma is stopped.