To remove reaction products adhering to the inner wall perfectly by disposing means for detecting the light from an illuminating means through a chamber at a position facing the illuminating means through the chamber.

A film deposition gas or a cleaning gas is introduced through a gas introduction port 14 into a chamber 11 from below and fed upward through the chamber 11 thence downward through the gap between chambers 11, 12 before being discharged through an exhaust port 15. An illumination means 21 and a light detecting means 22 are disposed oppositely while traversing the chambers 11, 12. Furthermore, a heater 13 is provided with a first window 23 for introducing the light from illumination means 21 into the chamber 12, and a second window 24 for introducing the light into the light detecting means 22 while traversing the chambers 11, 12. According to the structure, reaction products adhering to the inner wall of chamber 11 or 12 can be removed perfectly while monitoring the removed state thereof.