To provide a draft chamber that can efficiently prevent a leakage of a harmful substance outside to a work space from an opened top even in the case an obstacle exists in a front opening at the time of opening door and keeps an air flow good at the time of closing door.

A lower transverse sash 6 and vertical sash 7 composing a frame part of a door 2 are made of a hollow material and communicating with each other and a guide tube 10 facing downward and keeping upper end part attached to the top of a casing 1 is internally or externally inserted to the sash 7 in a manner that it can be slid air-tightly at the time of opening and closing the door 2 in up and down directions and air supplied to the guide tube 10 by blowing means 8 connected to the upper end part of the guide tube 10 is jetted to the casing through the hollow parts of the vertical sash 7 and lower transverse sash 6 from ventilation ports 16 and 17 formed in the lower transverse sash 6.