To prevent the sticking of scale on the inside wall surface in a circulation passage for water by applying magnetic field effectively on water.

A 1st flow passage 14 formed in a straight state from one end side of a flow passage 12 provided with a flow-in port 10a at one end and a flow-out port 10b at another end, a 1st reduced diameter part 16 communicating with the 1st flow passage 14 and having a diameter smaller than that of the 1st flow passage 14, a 2nd flow passage 18 communicating with the 1st reduced diameter part 16 and having a diameter gradually enlarged toward the direction of the flow-out port 10b and a 3rd flow passage 22 communicating with the 2nd flow passage 18 and forming the flow-out port 10b at another end side are formed in a device 10 main body as the flow passage 12. A valve body 28 having a diameter larger than the inside diameter of the 1st reduced diameter part 16 and arranged to be moved in the 2nd flow-passage 18 is provided in the 2nd flow passage 18. A stopper for restraining the valve body 28 at a position apart from the 1st reduced diameter part 16 by a prescribed distance at the time of moving the valve body 28 to the downstream direction by the pressing force of the water flow is provided. A magnetizing means 25 for applying the attraction force to the valve body 28 in the direction to attract to the 1st reduced part 16 is provided.