PURPOSE: To obtain an exact optical monitor value having reproducibility by detecting a clouding quantity of a window for an optical monitor by using a light beam whose wavelength is different from a wavelength which follows at the time of generation a film, and correcting the optical monitor value.
CONSTITUTION: A film generating device 1 is constituted basically of, for instance, a target 11, an anode 12, a high voltage power source 13, a gas leading-in port 14, and an optical monitor use window 2. A plasma light which is generated at the time of forming a film consists of a bright line spectrum peculiar to a target material and a leading-in gas. An optical monitor system 4 monitors and intensity of a light beam having a wavelength peculiar to the material, and checks whether a variation of a state, etc. exist or not at the time of forming the film. A reflection factor measuring instrument 5 measures a clouding quantity of the optical monitor use window 2 without interfering with a detecting light 3, by radiating and sensing an electromagnetic wave of a single wavelength being different from a wavelength of the detecting light 3. A correction coefficient calculating circuit 7 calculates an amplification factor from an output of the reflection factor measuring instrument 5, and an output value is corrected by varying an amplification factor of an amplifier 46a.