PURPOSE: To magnetically treat water with high efficiency by forming a magnetic field at right angles to a water current and narrowing the flow passage at the intersection between the magnetic field and the water current.

CONSTITUTION: Plural iron cores 1 are provided on the outer periphery of a duct 4 for a fluid 5 to be treated, and an exciting coil 10 is wound on the iron core 1. The iron core 1 is connected to the duct 4, and a tie iron core 2 with its end part 2A protruded into the duct 4 is arranged to form a narrow clearance 20 for passing the fluid 5. A means for activating the exciting coil 10 is further provided. Namely, a magnetic field is formed at right angles to the water current, the flow passage at the intersection between the magnetic field and the water current is narrowed, the electric field at the part can be enhanced, and hence water molecules can be efficiently oriented. Moreover, water can be treated to stabilized purity by regulating the intensity of the exciting coil regardless of the flow rate of the water to be treated.