The present invention provides a polymer membrane mainly made of an ion exchange resin which further comprises melt-spun fluororesin fibers such as fibers made of an ethylene-tetrafluoroethylene copolymer, a polyvinylidene fluoride or the like, having fiber diameters of from 0.01 to 20 .mu.m, fiber lengths of from 1 .mu.m to 10 mm and aspect ratios of at least 5 in an amount of from 1 to 40% based on the total mass. The polymer membrane is excellent in handleability and shows excellent dimensional stability when hydrated, and therefore a polymer electrolyte fuel cell provided with a membrane-electrode assembly having the polymer membrane as an electrolyte membrane has great durability.