To provide a draft chamber equipped with a wind speed controller keeping constant the speed of inflow wind flowing from the fully opened opening.

The draft chamber includes a body 11, a transparent shutter 14 opening and closing vertically a window section 13 of an operation chamber 12, an exhaust duct 20 connecting the operation chamber with outside air, an exhaust damper 23 regulating the exhaust air flow within the operation chamber and a drive motor 24 opening and closing automatically an exhaust fan 26 or the exhaust damper carrying out forced exhaust of air in the operation chamber through the exhaust duct. The chamber is equipped with a pair of opening/closing sensors 30 measuring the degree of the opening 15 of the shutter attached in a vertically movable way, a differential pressure sensor 37 measuring the exhaust air flow, a reset sensor 40 resetting the measurement of the opening/closing sensor obtained when the shutter passes through a predetermined height and a controller 46 treating the exhaust air flow of the exhaust damper or the exhaust fan by processing the signals from the sensors arithmetically to control an appropriate inflow wind speed at a constant level through the degree of the opening.