PURPOSE: To secure stabilized combustion state by a method wherein, solid contents, contained in flue gas, are collected and sent directly into a combustion furnace to throw them thereinto and burn them again.

CONSTITUTION: A conveyor tube 18, connected to a mixing machine 13, is communicated with a fuel supplying tube 5. Solid contents, separated from the exhaust gas and collected by a dust collecting machine 9, are supplied to the half way of the fuel supplying tube 5 by air transportation and are sent directly into a burner 6 together with solid fuel 1, crushed by a crusher 4, then, are thrown into a boiler 7. Accordingly, it becomes unnecessary to send them into a banker 2 through a bag filter whereby a phenomenon, such as the formation of a bridge in the banker 2 by the collected solid contents or the generation of slip in the crusher, which is caused in case a roll mill type crusher is utilized especially, may be eliminated.