PURPOSE: To provide the title apparatus for completely removing mist or the like by a mechanism wherein a zigzag duct having plural small pores at a corner portion thereof is arranged within a hermetically closed casing and a pressure regulating duct is connected to a terminal end portion of the casing.

CONSTITUTION: A duct 3 passing a treating gas is provided in a zigzag fashion within a hermetically closed casing 1 and an inlet and outlet 3a, 3b of the duct 3 are opened at a left and a right end surface of a casing 1. Moreover, an outlet casing 2 communicated with the duct 3 is provided at a right end surface of the casing 1 and a water drainage plate 4 is mounted at the vicinity of the outlet 3a. At a corner portion of a zigzag duct 3, a receiving wall surface 3c directed to the upstream of a gas is provided to receive a gas stream and plural small pores 9 are formed to a wall surface 3a. To the upper portion of an outlet side end surface of a casing, a pressure regulating duct 16 with a ralatively small diameter is connected and the other end of the duct 16 is connected to and communicated with an end portion of the outlet casing 2.