PURPOSE: To safely operate a dust collector without trouble by pouring noncombustible powder into the dust collector when detecting the unburned combustible dust coming flying from a combustor or detecting a temperature rise of waste combustion gas.

CONSTITUTION: Waste combustion gas from a combustor, such as a pressure fluidized bed boiler flows in a gas inlet chamber 6 from a gas introducing port 8 of a dust collector 1 and is distributed to every filter chamber 3. At this time, when the boiler is given stationary operation, dust contg. gas distributed to every filter chamber 3 flows down in the filter chamber 3 and is filtered on the inner surface to turn into clean gas. But when the combustion state of the boiler is not stabilized to be abnormal, large quantities of unburned combustible dust are discharged together with the dust and when the combustible dust is ignited and the like are detected, noncombustible powder 20, such as calcium carbonate is poured into the dust collector 1.