A membrane/electrode assembly for a polymer electrolyte fuel cell capable of exhibiting high power generation performance constantly for a long period of time in a high temperature and low humidity environment, and a polymer electrolyte membrane whereby such a membrane/electrode assembly is obtainable.A polymer electrode membrane 15, comprising a proton conductive polymer which has an electrical conductivity of at least 0.07 S/cm at a temperature of 80° C. at a relative humidity of 40% and which has a water content of less than 15 mass; and a membrane/electrode assembly 10 comprising an anode 13 and a cathode 14 each having a catalyst layer 11, and a polymer electrolyte membrane 15 disposed between the anode 13 and the cathode 14.