PURPOSE: To prepare a liquid fuel giving exhaust gas with decreased color and odor and having excellent combustibility, by enclosing the outer circunference of a pipe made of a ferromagnetic material with laminated iron plates connected to both poles of a permanent magnet, thereby applying a magnetic field of a specific strength to the fuel flowing through the pipe, and passing a direct current to the pipe.

CONSTITUTION: The pipe 1 is made of a ferromagnetic material such as mild steel. Laminated iron plates 2b and 2c are attached to both poles of a permanent magnet bar 2a to form a horseshoe magnet. The pipe 1 is inserted between the both ends 2d and 2e of the horseshoe magnet so as to contact the ends closely with the outer circumference of the pipe 1. The liquid fuel flowing in the pipe 1 is subjected to the magnetic field of 180W220 gauss by the arrangement. A direct current of 60W80mA is passed along the length of the pipe 1 by properly selecting the fixed resistor R connected to a battery 3. The treated liquid fuel is sent to a gasoline engine 10 by a fuel pump, and burnt.