A module filled with an organic porous ion exchanger is placed midway on a pipe for transporting high-purity water to be supplied to a point-of-use. As the organic porous ion exchanger is used an organic porous ion exchanger having a three-dimensional mesh structure with mesopores, having an average diameter of 1 m to 1000 m, in a wall between interconnected macropores and in which an ion exchange group is uniformly distributed and an ion exchange capacity is 0.5 mg equivalent weight/gram of dried porous body or greater.