PURPOSE: To improve the combustion efficiency of a fuel oil, to clean exhaust gas, to increase the efficiency of a low-grade fuel oil to that of a higher one, and to remove the scale or rust of water pipes in boilers, by changing the orientation of molecules in the fluid with a magnetic and an electric fields.

CONSTITUTION: A fuel oil, e.g. heavy or waste oil, is fed from the tank 6 to the pump 7, heated by the heater 8, and burnt in the burner 9. The fuel oil is subjected to the magnetic field produced by the unit 2 and the electric field produced by the unit 3 of the device 1 comprising the units 2 and 3 and the power source control units 5a and 5b simultaneously and/or in such a manner as to lag the electric field behind the magnetic field. For example, the magnetic field of 800 gausses and the electric field of 600 mW are applied to the fuel oil B flowing in the conduit to save the fuel by 16.9% and to give various advantages due to the change in molecular orientation of the fluid.