PURPOSE: To reduce water in chamber, by providing an ejector between a chamber to be exhausted and a water sealing pump thereby flowing inert gas containing less water than that contained in the atmosphere into the ejector.

CONSTITUTION: An ejector 2 is provided between a chamber 3 to be exhausted and a water sealing pump 1 to flow inert gas containing less water than that contained in the atmosphere into the ejector 2 and to discharge together with exhaust gas to the side of water sealing pump 1. Consequently, water in gas flowing in through the ejector 2 can be reduced resulting in reduction of water in chamber.