PURPOSE: To modify a hydrocarbon based liquid fuel in a very short time, improve thermal efficiency in combustion and reduce a fuel, by bringing permanent magnets having a very powerful magnetic field into contact with a fuel oil in a closed vessel, and modifying insoluble oily material molecules by magnetic resonance phenomenon.

CONSTITUTION: Permanent magnets 1 and 2 are mounted on iron yokes 3 in a closed vessel, and N poles are mounted on the same surface of one yoke 3. S poles are mounted on the same surface of the other yoke 3 and faced to attract to each other. The resultant assembly is then put into a casing 4. The number of the magnets depends on the flow rate of the passing fuel oil. The fuel oil is passed through the above-mentioned casing 4 to modify insoluble oily material molecules.