PURPOSE: To ensure stable combustion even at the time of low load operation by making the cross section of an air discharge port variable and compensating for the deterioration in the misting force of an atomizer at the time of low load operation.

CONSTITUTION: Fuel oil F passes burner gun 109 and is ejected from atomizer 110 in mist, forming droplets C. Combustion air sent from air duct 112 passes air box 105 and is given a rotational force by rotary vane 114 and then it is discharged from air exit 104. At the time of low load operation, the amount of oil is reduced and at the same time burner gun 109 is moved backward. By this, the conical surface of air adjusting cone 115 fitted at its end is brought close to the conical wall surface of air exit 104, and thereby the area of cross section of air exit 104 is narrowed to a proper area corresponding to the amount of air. By this, even at the time of low load operation, the combustion air is ejected at high speed, almost as high as at the time of high load operation, while ejected oil is being turned into mist and combustion continues as air is being mixed with oil droplets C.