PURPOSE: To perform complete combustion through combustion of a liquid fuel with a high efficiency, by a method wherein a magnetic field or an electric field is generated at a burner part, and this facilitates activation of the sprayed liquid fuel at the burner part and decomposition of it into a low carbide component.

CONSTITUTION: With a switch turned ON, an oscillator 2, a blower 5, and an ignition circuit 14 are activated. Petroleum in a spraying tank 1 is then irradiated with ultrasonic wave from an ultrasonic oscillator 3, while the petroleum, brought to a sprayed stated, is fed in a combustion case 9 after passing through a sprayed fuel conduit pipe 10 as it is mixed with the air fed from the blower 5, and the mixture is jetted ahead through combustion holes 12 in a burner part 11 for firing by means of an ignition plug 13 for combustion. A magnetic field as shown by a broken line is generated in the combustion hole 12, and the magnetic field is exerted on liquid drops 15 of the sprayed petroleum. Thereby, the magnetic moment of a high carbide component in the petroleum is activated, while the decomposition of the liquid fuel into a low carbide component is promoted to heighten a combustion speed and a flame temperature, resulting in the promotion of complete combustion of the liquid drops 15 and improvement of an efficiency.