To prevent an erroneous detection or a non-detection in the detection of an etching finish by a method wherein a detection signal which detects the emission intensity of light at a specific wavelength due to an etching treatment is compared with a reference signal and the amplification factor of a signal amplifier circuit is controlled by a comparison circuit.

The emission intensity of light 1 at a specific wavelength is detected by a photodetector 2, the difference between a detection signal by the photodetector 2 and a slice-level voltage are output by a differential circuit 3, and its output is amplified by a signal amplifier circuit 4. Then, the amplification factor of the signal amplifier circuit 4 is decided by a comparison output which is obtained by comparing the detection signal by the photodetector 2 with a reference signal, stored in a memory 8, by using a comparison means 7. Thereby, it is possible to prevent an erroneous detection or a non-detection in the detection of an etching end point, and an etching treatment can be performed stably.