One object of the present invention is to produce a weakly acidic cation exchanger under mild conditions. Another object of the present invention is to produce a more firm weakly acidic cation exchange film. Still another object of the present invention is to provide a weakly acidic cation exchanger capable of realizing high-level separation of monovalent cation and simultaneously analyzing monovalent cation and divalent cation and also provide a chromatography column using the ion exchanger. In the production method of a weakly acidic cation exchanger of the invention, a solvent incapable of dissolving a polymer having a double bond within the molecule is used and the weakly acidic cation exchanger is produced by polymerization at temperature of 100° C. or less.