Abstract: A method for forming a coating of an oxide on a support by the reactive sputtering technique, which comprises measuring the intensity of at least one spectral component having a given wavelength of the spectrum of a plasma formed between the support and a target composed of an oxidizable substance convertible to said oxide, comparing the measured intensity of the spectral component with the standard intensity of a spectral component of the same wavelength, and continuously or intermittently varying the physical amount of a sputtering gas and/or the amount of an electric current from a sputtering power supply so that the measured intensity of the former spectral component approaches the standard intensity of the latter spectral component; and a sputtering apparatus for performing the aforesaid method, which comprises a vacuum chamber, a target electrode disposed within the vacuum chamber, means for introducing a sputtering gas into the vacuum chamber, means for discharging the sputtering gas from the vacuum ch