PURPOSE: To allow quick evacuation without damaging a booster pump by opening a by-pass around the booster pump and closing it gradually under predetermined conditions during evacuation of a chamber by the booster pump and a mechanical pump.

CONSTITUTION: A load lock chamber 11 moves in processed material 19 through a load lock valve 13, and is communicated with a processing chamber 17 via a load lock valve 15. The processing chamber 17 is normally in an evacuation condition by means of a suitable pump means. An evacuation device formed by a tandem connection of a booster pump 21 and a mechanical pump 23 is provided to the load lock chamber 11. In this case, during evacuation of the chamber 11, a valve 45 in a by-pass 43 around the booster pump 21 is first opened, and the valve 45 is controlled to be closed gradually at a speed to maintain a differential pressure across the booster pump 21 at a predetermined level according to the gas pressure drop of the chamber 11.