To install permanent magnets easily in moisture-proofing and rust-preventing state and build in the permanent magnets in the middle of a pipe of an electrically conductive material by forming joining flange parts in both sides of the pipe and a small diameter part in the middle, installing permanent magnets face to face in the outer circumference of the small diameter part, and covering the outside of the magnets with moisture-proofing and rust-preventing synthetic resin.

In this apparatus 1 for treating a fluid, joining flange parts 3, 3 are formed in both ends of a pipe 2 made of a non conductive material of synthetic resin, a small diameter part 4 is formed in the middle part of the pipe 2, permanent magnets 5, 5 are installed face to face in the outer circumference of the small diameter part 4 while keeping the respective magnetic poles face to face, a pipe 6 with a large diameter is externally fitted, and the permanent magnets 5, 5 are covered with moisture-proofing and rust-preventing synthetic resin by filling the inside of the pipe 6 with the resin. Consequently, a fluid is treated by ion electric current generated when the fluid transversely passes at a prescribed flow speed or higher through the magnetic field generated in the inside of the small diameter part 4. For example, in the case of water treatment, the apparatus is installed in the middle of a circulating route formed in a water-receiving tank to carry out water which is constantly circulated and water scale and red rust are prevented from adhering to the inner wall of a water supply pipe from the water-receiving tank.