PURPOSE: To completely burn a solid fuel by a method in which a solid fuel is carbonized and burned in a primary combustion chamber and unburned gas generated is burned in a secondary combustion chamber.

CONSTITUTION: Fuel granules in a hooper 2 are sent in the direction of arrow 32 by a screw conveyer 6 by actuating a driver 31, directed into a primary combustion chamber 8 through a charging cylinder 7 as shown by arrow 38, and then stayed on a rostor 9. When firing an ignition burner 11, fuel on the rostor 9 is carnonized and volatile gases from the fuel are vaporized and sent through an exhaust cylinder 13 into a secondary combustion chamber 14, where air is sent by a secondary air blower 15 and the fuel is fired by the ignition burner 16. Hot gas produced by perfect combustion enters through a connection cylinder 17 into a boiler 18, is heat- exchanged there, and then discharged through an exhaust cylinder 19 to a stack.