PURPOSE: To enable insertion in a fuel tank of an automobile, and enhance combustion efficiency of liquid fuel by opposing an N pole and an S pole of permanent magnets to each other on both ends of an outer cylinder having a flow port, and arranging nonmagnetic substance balls between the N pole and the S pole.

CONSTITUTION: A flow port 12-6 is arranged in an outer cylinder 12-3, and plural powerful permanent magnets are fixed at regular intervals in the outer cylinder 12-3, and an N pole and an S pole of the permanent magnets 12-4 are arranged in the same direction. Nonmagnetic substance balls 12-5 such as ceramics are inserted between the N pole and the S pole of the powerful permanent magnets 12-4. The outer cylinder 12-3 is covered with a net 12-2, and both ends are calked and fixed by terminals 12-4. A magnetic field agitator 12 constituted in this way is inserted in a fuel tank 9 of an automobile 8, and travel is performed, and liquid fuel 14 is agitated by vibration of the automobile 8 and swinging of the nonmagnetic substance balls 12-5 between the powerful permanent magnets 12-4, and can be electrified by friction, and combustion efficiency can be improved.