Abstract: A fixed duct is formed by vertically setting a partition plate virtually in the middle of a duct with a U-shaped cross section composed of two side plates and a bottom plate in such a manner that the lower end of said partition plate is spaced from said bottom plate and by placing a canopy over the space between the upper end of one of said side plates and the upper end of said partition plate. An exhaust pipe is installed through said canopy. A sealing liquid is placed in said fixed duct in such a manner that the level of said liquid is always above the lower end of said partition plate. An S-shaped movable pipe extends from outside the upper end of the other of said side plates into said sealing liquid, further under the lower end of said partition plate, to above the level of part of said sealing liquid covered with said canopy. Said movable pipe freely moves along said fixed duct. Dust is collected at an end of said movable pipe and is sent to a dust collector through said exhaust pipe.