PURPOSE: To increase the utilization efficiency of the low ash content solid fuel and to prevent an umex pected condtion, such as going out, from occurring by supporting a solid fuel by a planner material, providing a cylinder encircling the solid fuel and forming a combustion chamber 4, and forming suction gaps of a narrow width between the lower end edge of the cylinder and the planar plate in proportion to the discharge amount.

CONSTITUTION: A cylinder 34 forming a combustion chamber 4 is provided at the upper position of a receiving plate 2 supporting a low ash content solid fuel 1. At predetermined positions of the cylinder 34, taper grooves 55 are formed and brought into engagement with pins 53. The cylinder 34 is rotated, lifted and lowered thereby to make it possible to adjust the width of suction gaps formed between ths same and the upper surface of a receiving plate 2. A firing material 11 is charged onto the receiving plate 2 and ignited and then a low ash content solid fuel 1 is charged therein, when combustion is started. When the combustion reaches a high temperature, the fuel is pulverized and flies, however, since the fuel 1 is supported on the planar receiving plate 2 which has no ventilation slits, even pulverized fuel does not fall but stays in the combustion chamber 4 of a high temperature to continue combustion. Further, with the combustion of the low ash content solid fuel 1, atmospheric air passes through the suction gap 36 and sucked up into the combustion chamber 4.