To reduce the weight of a balance weight of a draft chamber.

According to the structure of the draft chamber, a first rope suspending a lower door 3 is fixed to the balance weight 10. A second rope 14 suspends an upper door 4 via one end thereof, and the other end thereof has an elevating pulley 19 fixed thereto. The elevating pulley 19 has a third rope 20 wrap-connected thereto from above, and one end of the third rope 20 is fixed to the balance weight 10 and the other end thereof to a fixed body such as a main body 1, respectively. To maintain a balance between the upper door 4 and the balance weight 10, the balance weight 10 weighs only half the weight W2 of the upper door 4, and therefore the entire weight W3 of the balance weight 10 is equal only to (W1+0.5W2).