PURPOSE: To make possible the control of a remaining gas in the stage of film formation regardless of the condition in a vacuum chamber by coupling an analysis chamber having a high vacuum evacuation system via a pressure conversion orifice to the vacuum chamber and disposing a quadrupoll mass spectrometer in the analysis chamber.

CONSTITUTION: The analysis chamber 12 is communicated and coupled with the vacuum chamber 1 which is used to monitor the partial pressure of the remaining gas and a sluice valve 13 is provided between the chamber 1 and the chamber 12. The pressure conversion orifice 14 for the purpose of providing a pressure difference is provided between the chamber 1 and the chamber 12. The inside of the chamber 12 is evacuated to a high vacuum state by a high vacuum pump 15 and is provided with the quadrupole mass spectrometer 10. The pressure in the chamber 1 is converted to the pressure measurable with the spectrometer 10 by the orifice 14 and such pressure is introduced into the chamber 12 where the partial pressure of the remaining gas is measured by the spectrometer 10. As a result, the control of the remaining gas during sputtering which heretofore was impossible is made possible and the quality of the formed film is improved.