PURPOSE: To decrease the air ratio and the rate of oxygen content in exhaust gas, and to increase the combustion efficiency of an oil burner, by burning fuel oil after removing oil droplets of large diameter or the ones of large diameter and the middle-sized diameter, from the gas containing droplets having various diamter sizes.

CONSTITUTION: Oil is sucked by a pump 2 from a fuel tank 1, and sprayed into an oil droplet selecting pipe 5, swirling from a nozzle 4, as it is just sucked, or after it is preheated in an oil preheater 3. On the other hand, the air is fed into the pipe 5, swirling from a fan 9 at the end of an oil droplet selecting pipe 5, and large droplets of oil are centrifugally separated to the pipe-wall by the swirl air in the oil droplet selecting pipe 5. The centrifugally separated oil droplets are flowed down through the bottom of an oil droplet selecting pipe 5, being in the state of an oily film, and the dropped oil is circulated into an oil tank 1 through an oil separating pipe 7. The mixture of oil droplets, being fed from the other end of an oil droplet selecting pipe 5, and the air from an air feed pipe 8 is fed to be burnt into a combustion chamber 9.