To prevent the occurrence of combustion failure by suppressing growth of clinker.

Ceramic particles 30 are packed on a porous plate 4 to form a fire grate 3, and woody pellets 31 are dispersed on the fire grate 3. Combustion air is jetted from the lower side of the porous plate 4 for burning the woody pellets 31. On discharging ash occurring by the combustion to a downstream secondary combustion space by currents of combustion air or combustion gas, layers of the ceramic particles 30 and the woody pellets in the fire grate 3 are mixed by mixing means 21, 22 and clinker of the combustion ash formed in the fire grate 3 is cracked to suppress the growth of the clinker and prevent the occurrence of combustion failure.