PURPOSE: To reduce combustion ashes by decreasing fuel accumulating above a traveling grate which promotes combustion and gasification of fuel, before fuel falls on the traveling grate, forming a fuel falling zone positioned above a traveling grate.

CONSTITUTION: A burner 10 is formed with a high temperature thermal decomposition combustion chamber 25, by lining the surrounding walls 12, 14, 16, 18 and 20 with amorphous refractory material 22. An opening 39 for feeding the fuel is arranged at the horizontally extending insulator 24, and the fuel fed from the opening 39 falls into a primary burning chamber 25. A secondary air supplying path 76 is arranged to the inside of the primary burning chamber 25, and a fuel path from the primary burning chamber 25 to a secondary burning chamber 44 is formed with refractory blocks 28. A traveling grate 50 is arranged below the primary burning chamber 25 and after causing the fallen fuel at the primary burning chamber 25 to heap, the fuel is moved downward by a continuous belt 54. As a result of this, combustion and gasification of the fuel are promoted before the fuel falls, and the combustion ash is decreased.