PURPOSE: To accurately determine the end point of an etching irrespective of the maximum transmission wavelength of an optical fiber by determining the end point when an emitting light intensity out of a region in which the peak of the intensity is present is lowered as compared with the output of an emitting light spectrum during processing of a material to be etched.

CONSTITUTION: When an etching is started in a vacuum chamber 1 to generated a plasma discharge, a light having a wavelength of about wavelength λa in a range Q of a light having a spectrum is passed through a spectral filter 4. A photoelectric converter 5 receives the light passed through the filter 4, converts it into a level voltage (v) responsive to the received amount, and output it. When the etching is advanced to reach an etching end point, a decrease in the emitting light intensity of the discharge occurs in the whole range Q. Accordingly, the amount of the light passed through the filter 4 is reduced, and the converter 5 outputs the signal (v) responsive to the amount. A microcomputer 7 judges a decrease in the intensity of the discharge from a digital voltage signal V input at this time. The microcomputer 7 determines the time of the decrease in the intensity of the discharge as an etching end point.