PURPOSE: To enable a complete combustion of ashes and non-ignited materials to be performed by a method wherein a difference in height is made at a downstream side end of a tranferring direction of a combustion floor and a nozzle for air flow of desired energy toward the residue of combustion dropped at the difference in height is arranged.

CONSTITUTION: Residue of combustion 2a of solid material 2 ignited on a combustion floor 7 is dropped at the difference in height at an end part 7a of the combustion floor 7, the ashes and non-ignited materials having low specific weight is carried downstream on the flow of ignited gas G, non-ignited materials such as metal or the like having a high specific weight are dropped and recovered at the non-ignited material recovering part 9. A flow passage 11 is narrow and widened at the upper part of the non-ignited material collection part 10, so that the ashes and non-ignited materials are carried to the upper part of the non-ignited materials collection part 10 with the flow of the fast combustion gas G, thereafter they are dropped into the lower non-ignited material collection part 10. The ashes and non-ignited materials within the non-ignited materials collection part 10 have no non-combustible materials such as metal, so that a positive combustion is carried out and a thermal energy is supplied to the boiler chamber 12.