To provide a combustion device for extracting and utilizing thermal energy by continuously combusting a solid biomass fuel in a form of a formed particle or a pellet at over 800 °C, without using electric equipment such as a screw feeder or blower and ensuring secondary combustion, and permitting reduced device size, highly efficient combustion, and significantly increased calorific value, if such an electric device is used for this combustion device.

An opening part 11 is provided at the center of a bottom surface of a primary combustion chamber part 10 downwardly following a fuel cylinder 9 in the combustion device for downward combustion. A basket shaped three-dimensional grate 14 in which a basket is upside down to have a space upper the opening part 11 is placed over the opening part 11 for covering it. An inclining surface hearth furnace 15 of cone-shaped hollow is formed with respect to the opening 11 so that the pellet fuel can burn three-dimensionally. A necessary and enough number of combustion air holes 16 are provided on the inclined surface hearth furnace 15 and the wall of the primary combustion chamber part 10 to allow combustion flame F, combustion gas Ha and ashes G to enter a secondary combustion chamber part 12 through the basket shaped three-dimensional grate 14 and the opening part 11, where the combustion gas Ha secondarily combusts and the ashes G drop into an ash collecting part 13.