PURPOSE: To measure the gaseous pressure in a device without contacting by measuring the emission intensity of the specific chemical species in plasma.

CONSTITUTION: The device is constituted of an interference filter 5 which allows the transmission of light of, for example, 337nm wavelength, a photoelectric converter 6 which converts the intensity of the light transmitted therethrough to an electric signal and a recorder 7 which records the output A reactive gas (CF4+O2) 2 is passed in the plasma etching device 1 and an SiO2 film on a wafer 3 deposited with SiO2 on a Bi substrate is etched. The emission intensity of the gaseous nitrogen N2 in the leak gas is then monitored during the etching. More specifically, the fluctuation of the gaseous pressure on the wafer is exactly measured by monitoring the emission intensity of the N2 from the relation between the pressure during etching and the emission intensity of the N2. The uniformity of the etching and film formation is thereby evaluated and the production at a high yield is made possible.