PURPOSE: To decrease the size of a moving combustion means and to increase a moving speed, by a method wherein a fluidized bed is formed by a part of an ash content containing an unburnt content conveyed by the moving combustion means, and the unburnt content is burnt in the fluidized bed.

CONSTITUTION: By a moving combustion means 2, the unburnt content of fuel which is not completely burnt, is dropped toward a fluidized bed 7 of a fluidized bed combustion means 6, and the air for combustion is fed by a wind chamber 8 therebelow for complete combustion. This constitution eliminates the need to completely burn solid fuel in the moving combustion means 2 part, and enables increase of a fuel moving speed. Further, since the moving combustion means 2 itself can be decreased in size, the occurrence of unbalancing combustion on the upper stream and the downstream is reduced, resulting in the possibility to decrease the coefficient of excess air.