PURPOSE: To obtain a good-quality film with good reproducibility by a method wherein a window to be used to observe a beam of light emitted in a reaction region is installed in a reaction chamber and a spectroscopic monitor is connected to this window in order to observe a discharge state and to enhance the controllability of film formation.

CONSTITUTION: A window 8 to be used to observe plasma beam is installed in a reaction chamber 1. A spectroscopic monitor 9 to be used to analyze the plasma beam is connected to the window 8. A discharge state inside the reaction chamber 1 is observed by using the monitor 9 through the window 8. A measured signal of a beam of a specific wavelength obtained by using the monitor 9 is input to an error amplifier 10. The amplifier 10 compares the measured signal with a preset signal; an electric power value of microwaves, the amount of a reaction gas and the like to be introduced into the reaction chamber 1 are feedback-controlled. By this setup, the controllability of film formation is enhanced; a good-quality film can be obtained with good reproducibility.