PURPOSE: To achieve longtime continuous use and accurate measurement, by providing an air flow velocity sensor for detecting the bypass air flow velocity Vs of the air sucked into an exhaust dust in a bypass part provided to the exhaust duct so as to be communicated with the air.

CONSTITUTION: The gas generated in a draft chamber main body 1 is exhausted from an exhaust duct 3 through an exhaust port 17 and a bypass air duct 19 the air flowed into said draft chamber main body 1 at appropriate inflow wind velocity from a front opening part 9. At this time, gas with low specific gravity is flowed into the exhaust duct 3 through the exhaust port 17 while gas with high specific gravity is flowed into the exhaust duct 3 through the bypass air duct 19. By the exhausion from the exhaust duct, 3 the air is sucked into the exhaust duct 3 from a bypass part 5 and the bypass wind velocity V3 of the air when the air is sucked into the bypass part 5 is detected by an air flow velocity sensor 7. The bypass flow velocity Vs is converted to the exhaust amount Q of the exhaust duct 3 by an operation apparatus.