PURPOSE: To maintain a processing chamber always to be clean by opening a shut-off valve for preventing the backflow of oil vapor together with performing exhaust when the pressure difference between the processing chamber and an oil-sealed rotary vacuum pump is large, and closing the shut-off valve for preventing the backflow of oil vapor when the pressure difference is nearly zero.

CONSTITUTION: On the exhaust pipe 2 of a process tube 1, an oil-sealed rotary vacuum pump 3 and a mechanical booster 4 are mounted therebetween to be able to perform vacuum exhaust. And between the tube 1 and the booster 4, an automatic control device 5 and, sandwiching that, a pair of shut-off valves 6, 7 are mounted. Again, on the upper course side of the upper stream side shut-off valve 6 and on the lower course side of the lower stream side shut-off valve 7, vacuum pressure gauges 8, 9 are mounted respectively to be able to measure the vacuum pressure in exhaust pipe 2 respectively. Still more, each of vacuum pressure gauges 8, 9 is connected to a controller 10, and thereby controls the lower stream side shut-off valve 7 in opening and closing. That is, when the pressure difference between respective vacuum pressure gauges 8, 9 exceeds the prescribed value the shut-off valve 7 is opened and when it is less than the prescribed value, the valve is closed.