Process for production of electrolyte membrane and solid polymer type fuel cell

An electrolyte membrane characterized by comprising a porous body formed from a melt-moldable fluororesin and having interconnected pores and an ion exchange resin with which the interconnected pores are filled, is provided. Specifically, the porous body is formed, for example, from an ethylene/tetrafluoroethylene copolymer, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, a tetrafluoroethylene/hexafluoropropylene copolymer, a tetrafluoroethylene/ÄCF2=CF-(OCF2CFY)a-Oc-(CF2)b-SO3HÜ copolymer (wherein Y is a fluorine atom or a trifluoromethyl group, a is an integer of 0 to 3, b is an integer of from 0 to 12, and c is 0 or 1, provided that when b=0, C=0), or the like. An electrolyte membrane reinforced by such a porous body has high mechanical strength even if it is thin, and is excellent in dimensional stability when hydrated, and a polymer electrolyte fuel cell with a membrane-electrode assembly having the electrolyte membrane provides a high output and is excellent in durability.