To suppress the generation of clinker so as to prevent the generation of a burning trouble, ceramic particles 30 are covered over a porous plate 4 to form a fire bed 3. Woody pellets 31 are scattered on the fire bed and combustion air is ejected from the lower side of the porous plate so as to burn the pellets. At the time of discharging ash generated by the burning to a secondary combustion space on the downstream side together with an air current of the combustion air or combustion gas, the ceramic particle layers and the woody pellet layer are stirred by a stirring section 21, 22 to thereby break clinker of the combustion ash formed in the fire bed, whereby the growth of the clinker is suppressed and the generation of a burning trouble is prevented.