PURPOSE: To obtain a combustion apparatus of a low price and of high efficiency, by employing instead of a fire grate, a fire bed consisting of a flat board of metals, concrete etc. covered all over thereof with ash, volcanic ash, earth etc., and providing a combustion tube on top of the inside of a radiation tube.

CONSTITUTION: After being thrown through a fuel throwing-in ports, at the same time a fire door, 2D onto a fire grate 4 consisting of a flat board of ordinary stones, metals, or concrete covered all over thereof with wood ash, coal ash, earth etc., and ignited, oil coke is heated to a red heat and becomes live coal. Further, from the upper part of a combustion tube 3 fresh air and oil coke flows down to the lower part inside the combustion tube 3, contacts the live coal there, where said oil coke becomes white-hot and burns. Heat flow thus generated flows mainly out of a firing port 3D, then flows from the lower part toward the upper part along the flow of the fire grate 4, circulating along the inside wall of a radiation tube 2, then to discharge outlet 2F.