PURPOSE: To prolong the life of a mist trap and vapor trap by constituting the mist trap and vapor trap in such a way that the entire region of the packings therein contributes effectively to the capturing of vapors and mists.

CONSTITUTION: The operating temps. of both of a mist trap 3 and a vapor trap 4 are controlled simultaneously in accordance with the pressure drops of the respective traps in such a way that always a stable operating condition is obtd. More specifically, a differential pressure gage 11, an air flow control damper 12, and a controller 13 for heating are provided to the trap 3, and a differential pressure gage 14, an air flow control damper 15 and a controller 16 for heating are provided to the trap 4 as well. The output signals of the gage 11 and the gage 12 are inputted to a calculator 17. The respective dampers 12, 15 and controllers 13, 16 of the trap 3 and the trap 4 are made controllable by the output signal from the calculator 17.