POLYMER ELECTROLYTE MEMBRANE, MEMBRANE ELECTRODE ASSEMBLY USING SAME, AND SOLID POLYMER FUEL CELL

To provide a polymer electrolyte membrane that has excellent proton conductivity even under low-humidification conditions, has excellent mechanical strength and chemical stability, and can achieve high output and excellent physical durability when made into a polymer electrolyte fuel cell. The polymer electrolyte membrane includes a block copolymer containing each one or more of: a segment (A1) containing an ionic group; and a segment (A2) not containing an ionic group. The polymer electrolyte membrane forms a co-continuous (M1) or lamellar phase-separated (M2) structure, and gives a crystallization heat quantity of 0.1 J/g or larger determined by differential scanning calorimetry, or a degree of crystallinity of 0.5% or larger determined by wide-angle X-ray diffractometry.