To provide a cation-exchange resin inhibiting residue of impurity and generation of decomposed material, and having scarce eluting material.

The cation-exchange resin is produced by conducting sulfonation to a cross-linked copolymer having a specific eluting compound content of not greater than 400 g to 1 g of the cross-linked copolymer when obtaining the cross-linked copolymer by copolymerizing a mono-vinyl aromatic monomer with a cross-linkable aromatic monomer and producing a cation-exchange resin by conducting sulfonation to the obtained cross-linked copolymer. The cation-exchange resin having a reduced residue of impurity and scarcely generating eluting material when in use can be obtained since a specific eluting compound such as an eluting polystyrene is fixed at the step of polymerization.