The present invention relates to a sulfonic acid group-containing polymer excellent in ion conductivity and durability, a method for producing the same, a resin composition containing the sulfonic acid group-containing polymer, a polymer electrolyte membrane, a polymer electrolyte membrane/electrode assembly, and a fuel cell. The sulfonic acid group-containing polymer of the present invention, in a first embodiment, includes a constituent represented by the following chemical formula 1: ##STR00001## wherein X represents hydrogen or a monovalent cation species; Y represents a sulfone group or a ketone group; and n represents an arbitrary integer not less than 2.