To achieve a better atomization of a liquid fuel, eventually a better combustion thereof using a limited amount of air instead of a relatively large amount of air that will cause an adverse effect on the combustion in the gaseous fuel powered operation in a burner suitable for the both operation by a gaseous fuel and the liquid fuel.

A liquid fuel 4 is directed at a high flow and swirl velocity into settling chamber 13 of an air blast nozzle 2, where the flow and swirl velocity is reduced to form a thin fluid film on a prefilming lip 15. Blast air 5 from a blower is sent to rub the film and separates small fuel droplets with a very small particle size from the fuel film to be supplied to a burner inner chamber together with air.