Weakly acidic carboxylic resin, with no sulfur atoms, was selected as an alternative to sulfonic cation exchange resin for water treatment in nuclear power plants. Pyrolysis experiments showed that no corrosive SOx gas was produced by the former, and its residual ratio was one-third that of the latter conventional resin. Spent resin treatment then becomes relatively simple for the new resin. Subsequently, filtration characteristics were examined by use of simulated condensate water, assuming that the resin was applied to a filter demineralizer. The resin lifetime was ∼1.5 times that of the conventional one, suggesting that the amount of spent resin generated from a filter demineralizer could be reduced to about two-thirds. Therefore, the carboxylic resin showed favorable features for both water purification and spent resin treatment.