PURPOSE: To reduce the weight of a treating device by using directly a permanent magnet as a core, instead of using a steel plate yoke as magnetic poles in constructing a magnetic field treating device which gives a magnetic field to fuel and improve its combustion efficiency.

CONSTITUTION: An enclosure is formed by mild steel plates 3, 4 and 5 having no residual magnetism. Two opposing mild steel plates 4 are fitted each with permanent magnet 1 of the same plane area as the mild steel plates by utilizing the attractive force of said magnet. Magnets 1 are anisotropic. They are arranged so that their N and S poles face each other in the direction in which the magnetic field is set up, and thereby a magnetic field is produced between these magnets. The magnet is placed in a fuel path and thereby a magnetic field is given to fuel.