PURPOSE: To enable uniform-speed suction even if the velocity of flow in an exhaust gas duct changes by providing a sealing mechanism which makes sealing with dust at the intermediate point of a dust return line and providing an air quantity regulating mechanism on the upstream of a suction blower.

CONSTITUTION: The exhaust gas by combustion of coal passes the inside of the exhaust gas duct 1 from a boiler body and the duct in the exhaust gas is sucked together with the exhaust gas from a sampling nozzle 2 into a sampling line 4 and is supplied to a capturing cyclone 5. The supplied gas is separated to the dust and the exhaust gas in the cyclone 5 and the captured dust is transported via an ash hopper 14, an unburned component measuring instrument 6, and a fixed rate feeder 7 to a hopper 15. The dust in the hopper 15 seals the dust return line 8 by a seal feeder 16 and is sent to the return line 12, thereafter, the dust is sent by the discharge flow of a suction blower 11 and is returned to the duct 1 by the exhaust gas of the exhaust gas return line 10 separated in the cyclone 5. The suction of the exhaust gas is executed by a blower 11 and the air quantity is regulated by an air quantity regulating valve 17.