A membrane-electrode assembly for solid polymer electrolyte fuel cells, in which a sulfonic acid is an ion exchange group, and the heat resistance is superior, is provided. The membrane-electrode assembly for solid polymer electrolyte fuel cells contains a polymer including a principal chain that forms polyphenylene structure, a branched chain having a sulfonic acid group, and a branched chain having a nitrogen-containing heterocyclic group. It is preferred that a branched chain having a nitrogen-containing heterocyclic group is a structure expressed by the general formula (D) below. ##STR00001##