PURPOSE: To form a sputtered film with low residual stress by providing an auxiliary evacuation system furnished with a differential-pressure chamber separately from the main evacuation system for evacuating a vacuum sample chamber to precisely control the gas pressure in the vacuum sample chamber.

CONSTITUTION: The inside of the vacuum sample chamber 1 is roughly evacuated to a specified pressure with an oil rotary pump 24 by opening a rough evacuation valve 21. The valve 21 is closed, an auxiliary valve 23 is opened, a variable main valve 22 is opened, and the inside of the vacuum sample chamber 1 is finally evacuated by an oil diffusion pump 28 through a liquefied nitrogen trap 26 and a water-cooled baffle 27. After the fact that the degree of vacuum in the sample chamber 1 is controlled to a specified value is confirmed, the variable main valve 22 is closed, and a specified amt. of an inert gas in introduced into the sample chamber 1 from a gas inlet valve 7. A stop valve 30 is opened while monitoring the gas pressure in the sample chamber 1 with a vacuum gage 8. The inside of the sample chamber 1 is kept at a specified gas pressure by the oil diffusion pump 28 through the auxiliary evacuation system 29, the stop valve 30, the liquefied nitrogen trap 26, and the water-cooled baffle 27.