PURPOSE: To obtain a correct measured value with a higher S/N, by arranging optical filters placed within a light incidence section, one for cutting off short wavelength range and the other for cutting off light transmitting radio waves.

CONSTITUTION: Light to be measured of a plasma entering at an incidence window 1 which serves as filter for cut off short wavelength range while vacuum sealing up the inside of a detector section is transmitted through filters 2 and 3 arranged sequentially separated from the incidence window 1 and detected with a light receiving element 4 placed behind the filter 3. The filter 2 is made up of a copper mesh deposited on a sapphire plate for cutting off short wavelength range and shall have a sufficient transmission factor for the light being measured in the measurement of plasma and a reflectance enough to cut off noise signals with the wavelength larger than the light being measured. The incidence window 1 and the filter 3 have a sufficient transmission factor for the light being measured in the measurement of plasma and a reflectance enough to cut off noise signals with the wavelength less than the light being measured. This can interrupt the incidence of light with the wavelength less than about 50μ into the light receiving element 4 and the mixing of radio waves when the light receiving element 4 is used as an antenna.