PURPOSE: To reduce particularly kinematic viscosity, improve spraying characteristics and combustion efficiency and prevent effectively the generation of black smoke, by applying magnetic force from plural magnets to heavy oil passing through a channel.

CONSTITUTION: Heavy oil flowing from a joint part 11 is passed through a channel 13 and discharged from a joint part 12. Magnetic force of plural magnets 2 provided along the channel 13 to face each other with the channel 13 between is applied to the heavy oil during the passage through the channel 13 to promote the formation of ultrafine particles of molecules composing the heavy oil. Thus, the kinematic viscosity of the heavy oil is reduced.