Magnetic field generation device 5a and 5b, for applying a magnetic field to water, are located in the vicinity of an inlet 7 or an outlet 8 in a water passage area wherein pairs of electrodes 11 and 12 are arranged with intervening gaps. In the water passage area wherein the electrodes 11 and 12 are disposed, passage walls 10, which are arranged at predetermined intervals and which are perpendicular to the axial direction of a pipe line 2, define water passages, and, with the internal surface of the pipe line 2, openings 10a and 10b. The electrodes 11 and 12 are mounted on the faces of the passage walls 10.