To constitute the front end part of a working table of a draft chamber so as to prevent a tube and a cord connected to equipment used for working from obstructing.

In the draft chamber, a rectifying plate 31 installed at the front end part 4a of the working table 4 is rotatably held, and the tube 36 and the cord 35 are held by opening the rectifying plate 31, thereafter the rectifying plate 31 is closed, thereby enabling holding them so as to prevent the tube 36 and the cord 35 from obstructing in working of workers.