PURPOSE: To achieve that a plasma light quantity extracted from an etching treatment chamber is made always definite when an etching end point is detected and to stably detect the etching end point in a semiconductor-substrate dry etching apparatus.

CONSTITUTION: Plasma light generated from an etching treatment chamber 1 is transmitted through a light-shielding plate 3 and reaches an etching end point detector 4. The plasma light is converted into an electric signal by an optical/electric converter 5. The electric signal is taken into a position control part 6, controls a driving mechanism part 7 and drives the light-shielding plate 3 to the up-and-down direction. Thereby, optimum plasma light reaches the etching end point detector 4 and the etching end point detection of the title apparatus is stabilized.