PURPOSE: To perform excellent combustion, by a method wherein, a gun type burner having a spray nozzle, an air pass-through distance on the downstream side within a combustion chamber is lengthened to cover the spray pattern of the fuel, and combustion is brought to a blue flame state.

CONSTITUTION: Where the cross sectional area of an air flow passage 3 is AA and the cross sectional area of a combustion chamber 5 is AL, a ratio between the two cross sectional areas is set to AA/AL≥about 0.2, and the sizes of air nozzles 6a, 6b, and 6c of the combustion chamber 5 are increased in order noted above. In such constituted combustion device, when the air for combustion is fed through the air flow passage 3, such air flows into the combustion chamber 5 through the air nozzles 6a, 6b, and 6c. In this case, the size of the air nozzle of a point in the combustion chamber increases as approaching to the downstream side 10c of the combustion chamber 5, and thereby the flow-through distance of the air on the downstream side within the combustion chamber 5 is lengthened. With this structure, when an ignition plug 9 is actuated to inject the fuel through a spray nozzle 8, blue flame combustion is effected simulataneously with ignition.