PURPOSE: To shorten a time taken for drawing gas out of a reactor by restraining the speed of gas passing through a gas filter cloth within an allowable extent while preventing the cloth from breakage.

CONSTITUTION: When a flow regulating valve 3 is opened, gas in a reactor 1 enters a gas filter 2 through a piping 6 and further ejected through a piping 7. Pressure in the gas filter 2 is detected by a pressure gauge 8. An amplifier 9, flow meter 5 and flow regulation meter 4 cooperating with each other throttle the opening of the valve 3 when the internal pressure in the gas filter is small and enlarge the opening of the valve 3 when the internal pressure in the gas filter is large. Pressure in the reactor 1 is detected by a pressure gage 10, and a set value of the gas filter pressure corresponding to the internal pressure in the reactor is obtained from a function generator 11. The pressure regulation meter 12 regulates the opening of the pressure regulating valve 13 so that the internal pressure in the gas filter is the set value of the gas filter pressure. As a result, when the internal pressure in the reactor is large, the internal pressure in the gas filter is heightened, the opening of the valve 3 is enlarged and degassing can be quickly carried out.