PURPOSE: To restrict increasing of soot and dust as well as to improve an efficiency of combustion by a method wherein only unburnt solid material having large diameter particles is burnt again in a combustion furnace 1.

CONSTITUTION: Unburnt material 17 is separated into large diameter particles and small diameter particles with a fluidized bed type classifier 3, the separated small diameter particles 18 are removed and the large diameter particles 19 are fed to a feeding tank 4. The large diameter unburnt material 20 in the feeding tank 4 is fed to a combustion furnace 1 under an air feeding system. Feeding amount of unburnt material 20 is adjusted by increasing or decreasing a volume of feeding air under a variation of degree of opening of a damper 12. Feeding volume of solid fuel 15 is controlled in such a way as a hearth bed temperature 9 becomes constant in order to make a stable output of boiler etc. in case of the fluidized bed type furnace. Feeding amount of unburnt material 20 with large diameter particle is controlled in response to an increased or decreased storing volume in the storing tank 4 independent of the hearth bed temperature 9. Thereby, it is possible to restrict an increased volume of soot and dust and at the same time to improve an efficiency of combustion.