PROBLEM TO BE SOLVED: To provide a water treatment method and device in a nuclear power plant which reduce oxidation accelerating substances in the water to be treated before demineralizing the water to be treated including the oxidation accelerating substances caused by the radiolysis of water in a nuclear power plant by an ion exchange resin and maintain the quality of the treatment water at high purity by reducing a load of a demineralizer, and extend the life of the ion exchange resin and reduce a generated amount of the spent ion exchange resin.SOLUTION: The water to be treated including the oxidation accelerating substance generated by the radiolysis of water in the nuclear power plant is brought into contact with a filter having an ion exchange functional group introduced by radiation graft polymerization on a polyolefin non-woven fabric and metallic oxidation fine particles loaded on a part of the ion exchange functional group so as to decompose the oxidation accelerating substance, and subsequently b
