PURPOSE: To maintain the performance of an air curtain irrespective of opening/ closing conditions of a door to normally and surely block an opening.

CONSTITUTION: A circulating blower 10 for introducing outdoor air is installed in the rear of the upper half part of a draft chamber body 1. On the delivery side of the circulating blower 10, an intake pass 11 consisting of a branch pass 11a extending to the front side of the draft chamber body 1 and a branch pass 11b communication with a treatment chamber 4 is connected. In the draft chamber body 1, a hollow door 8 provided with a pair of glass plates and a bottom plate having a jet hole 28 which slides vertically along its front to open and close an opening 2 is installed. After outdoor air introduced by the circulating blower 10 is fed into a door housing space 9 through the branch pass 11a, an air chamber 34 and ventilation slots 12, it is conducted to a hollow part of the door 8 from the upper end of the door 8 and jetted downwards from a jet hole 28 at the lower end of the door 8 in the uniform air volume distribution and at an increased wind velocity to form an air curtain.