To provide a magnetic water activator whose case bodies can easily be mounted even in the inside of a narrow space such as a city water meter box.

The magnetic water activator is provided with: a pair of separated case bodies 2 fitted to the outer circumferential part of a water pipe 5; and magnet storing bodies 3 where each side in contact with the water pipe 5 is provided with permanent magnets 33, and an L-shaped projection 23 and an L-shaped recessed part 24 engaging the separated case bodies 2, respectively by sliding the case bodies 2 to the longitudinal axial direction of the water pipe 5 are provided. Then, in the case the magnetic water activator 1 is fitted to the water pipe 5 in the city water meter box 6, the separated case bodies 2 are inserted into a gap between the water pipe 5 and the city water meter box 6, and the case bodies 2 are arranged in a confronted state. Then, the L-shaped projection 23 is inserted into the insertion part 24b of the L-shaped recessed part 24, and the case bodies 2 are slid to the axial direction of the water pipe 5 so as to be engaged.