To decrease the generation of NOx in combustion gas.

Petroleum is flied in a combustion chamber 9, surrounding a gasifying cylinder 16, by a centrifugal force through a dispersion nozzle 16 at the lower part of the gasifying cylinder 16 for combustion, petroleum is gasified by the increase of the temperature of the gasifying cylinder 16, and injected through a burner port 6a through a gas chamber 7 for combustion. In a so formed burner, a wide air interruption ring 8 is situated below the burner port 6a group. After a flame F of incomplete combustion long from the burner port 6a to the vicinity of an inner peripheral edge 8b of the air interruption ring 8 is formed, complete combustion is effected on the central side. Since a combustion completion time is delayed, the flame is lengthened, the average temperature of the flame is low, and the generation of NOx is reduced.