In one aspect of the invention, CHF.sub.3 gas and CF.sub.4 gas (i.e., reactant gases), and argon gas (i.e., plasma-stabilizing gas) are introduced into a vacuum chamber. RF power is then applied between the electrodes within the chamber, thereby generating plasma. The plasma is applied to a substrate placed in the chamber, thus etching the SiO.sub.2 film formed on the substrate. A spectrometer extracts a light beam of a desired wavelength, emitted from the CF.sub.2 radical which contributes to the etching. An end-point detecting section monitors the luminous intensity of the CF.sub.2 radical reacting with SiO.sub.2 during the etching. Once the SiO.sub.2 film has been etched away, the luminous intensity of the CF.sub.2 radical increases. Upon detecting this increase, the section determines that etching has just ended. The selected wavelength ranges from 310 nm to 236 nm, preferably being 219.0 nm, 230.0 nm, 211.2 nm, 232.5 nm, or any one ranging from 224 nm to 229 nm. In another aspect of the invention, the device attached to the observation window of the chamber removes products stuck to the window during the etching. The window thus cleaned, more light than otherwise passes through the window and reaches the spectrometer. This enables the section to detect even a slight change in the luminous intensity of the CF.sub.2 radical, thereby detecting the end point of etching with accuracy.