A method of combusting a coal-water mixture fuel. The mixture is atomized into a conically-shaped primary pre-combustion chamber. Primary air is supplied, at a rate smaller than that required for complete burning of the mixture and in the form of a swirl about the axis of the jet of the atomized mixture, from the peripheral portion of the primary pre-combustion chamber into a secondary pre-combustion chamber connected to said primary pre-combustion chamber, thus forming a region of low pressure around the jet of the mixture. The region of low pressure serves to induce the atmosphere gas from the secondary pre-combustion chamber of higher temperature back into the primary pre-combustion chamber of a lower temperature, so that the water content of the mixture is evaporated and the mixture is ignited by the heat of the hot atmosphere gas. Then, the mixture is burnt in the secondary pre-combustion chamber in the presence of the remainder portion of the primary air, at a comparatively small air-to-ratio, so that a region of a reducing atmosphere is formed to suppress the generation of NOx. Then, secondary air is supplied, in the form of a swirl about the axis of the jet of the mixture and at a rate sufficient to completely burn the mixture, into the furnace connected to the secondary pre-combustion chamber, thus completely combusting the coal-water mixture.