PURPOSE: To provide an apparatus which gives an alarm or adjusts the opening height of a front door when exhaust gas is not appropriately discharged by measuring the opening height of the front door of a local exhaust chamber and the wind velocity within an exhaust duct and judging on the basis of data obtained by the above measurement whether a local exhauster is appropriately discharging the exhaust gas or not.

CONSTITUTION: A local exhaust chamber 11 is provided with a front door 12 for opening and closing the local exhaust chamber 11, an opening height measuring system for the front door comprising a potentiometer 17 for measuring the opening height of the front door 12 and an exhaust duct system 19 for exhausting the local exhaust chamber 11. An in-duct wind velocity measuring system comprises an anemometer 20 for measuring the wing velocity in the exhaust duct 19 and the like. A judging function comprising a computer 21 checks whether the difference between suction air volume (X) and discharged gas volume (Y) satisfies preset conditions, wherein the suction air volume (X) is obtained as the product of the width (w) of the front door, the opening opening height (h) of the front door and controlled wind velocity (v) in the vicinity of the front door 12 and the discharged gas volume (Y) is obtained as the product of the section area (S) of the exhaust duct and the wind velocity (V) within the exhaust duct.