A polymer electrolyte includes a heat-treated polymerization product of a polyurethane-based compound and a polyethylene(meth)acrylic acid, wherein the polyurethane-based compound is produced by polymerizing a diisocyanate-based compound, a phosphoric acid-based polyol, and a chain extender. The polymer electrolyte has a high ionic conductivity at high temperatures without causing deformation of an electrolyte membrane. The polymer electrolyte membrane can be inexpensively and simply manufactured, and the thickness of the membrane can be easily controlled. In addition, a large amount of phosphoric acid can be impregnated into the polymer electrolyte. A fuel cell that is operative at a temperature of 100.degree. C. or higher under non-humidified conditions and has improved energy generating efficiency can be prepared by employing the polymer electrolyte membrane.