To remove dust effectively by blowing fresh air from a dust collecting nozzle when a temperature detected value exceeds a set value after a specified time has been passed after a damper used for a pipe duct for feeding waste heat gas exhausted from a cement kiln to a waste heat boiler has been closed.

When waste heat gas containing dust flows from the upstream side of a pipe duct, dust is adhered to the shaft 11 of a damper and a blade 10 and, in particular, much dust 16 is accumulated around a valve seat 13. When dust 16 is accumulated, the blade 16 of the damper is closed imperfectly, and there occurs a problem that hot waste heat gas leaks and flows to the downstream side. To prevent this problem, after a specified time has been collapsed after the damper has been closed, the detected temperature of thermosensor set on the downstream side of the damper is compared with a set value and, when it is above the set value, the damper is judged to be closed imperfectly, and the solenoid valve 21 is opened, and fresh air is introduced from a dust collecting nozzle 20 around the valve seat 13 so as to blow off accumulated dust 16.