PURPOSE: To increase the endurance time of combustion of fuel per unit volume remarkably and burn it at a high temperature while reducing the generation of nitrogen oxide even when the fuel, having a low calorific value such as alcohol or the like, is employed.

CONSTITUTION: Primary fuel gas, generated by burning liquid fuel such as alcohol or the like in the fuel layer 20 in a combustion chamber 18, is mixed with pressure air, ejected out of an air nozzle 16, in a gas downflow pipe 14. In this case, whirling current is formed by ambient air, rolled into the ejected air, and eddy current is generated in a flow passage sectional area changing site by contracting the flow passage sectional area of the primary fuel gas compared with the flow passage sectional area of ejected air from the air nozzle 16, then, expanding it whereby hydrocarbon in the primary fuel gas is decomposed into carbon and hydrogen, high in activity, due to the reaction between the primary fuel gas and air and pressure air from a pressure air injection port 46 is mixed into the decomposed carbon and hydrogen, then mixture is ignited by an ignition device 38, thereby permitting the obtaining of combustion, high in temperature, long in the time of endurance and reduced in the discharge of nitrogen oxides.