PURPOSE: To improve combustion properties of fuel oil by using both cavitation and static electricity, by introducing the fuel oil into a casing with a built-in high-speed rotating rotor, forming an uneven state with a nearby face each other.

CONSTITUTION: A fuel such as heavy oil is introduced from the inlet pipe 11 into the casing 2 wherein the rotor 3 is rotated usually at 500W3,600 revolutions/min. The fuel is introduced into the gap between the casing having a ridgy shape in the longer direction of the casing 2 and the length between the top and top of the ridge of usually 0.1W5mm (preferably 0.5W1.5mm) and the cylindrical rotor 3. In the operation, cavitation and static electricity by friction are generated in the oil, and the composition substance of the oil is processed into dispersed particles and made into an excited state, to improve the fuel oil.