PURPOSE: To improve the efficiency of combustion by acting a magnetic field on fuel which flows through a fuel system.

CONSTITUTION: A permanent magnet device 7 is provided in a fuel passage 6, which connects a fuel pump chamber 3 to the float chamber 5 of a carburetor 4, in a position near the float chamber 5. In this permanent magnet device 7, a board type permanent magnet 16 is fitted and fixed in a nearly cylindrical yoke 15, forming two fuel passages 17, which have trapezoidal cross sectional area being partitioned by the permanent magnet 16. A hose mouth piece is screwed in each of the both end parts of the yoke 15 respectively, and the fuel tubes 6a of the fuel passage 6 connecting a fuel pump to a carburetor float chamber, are fitted in these mouth pieces 18 respectively. Thereby a good combustion can be provided, improving the fuel cost.