The invention discloses a polymer electrolyte which comprises, as the main component, a block/graft copolymer comprising, as constituent components, polymer blocks (A), (B) and (C) which cause phase-separation from one another, wherein the polymer block (A) comprises a vinyl compound unit as the main repeating unit and has an ion-conductive group, the polymer block (B) comprises a vinyl compound unit capable of forming a flexible phase as the main repeating unit and forms a flexible phase, and the polymer block (C) comprises a styrene derivative unit carrying an alicyclic hydrocarbon group having a polycyclic structure as the main repeating unit and forms a restrained phase; a membrane; a membrane-electrode assembly; and a solid polymer fuel cell. The polymer electrolyte has excellent durability and heat resistance, and shows little change in properties, such as the change in dimension between a dried state and a wet state, the change in mechanical properties and the change in methanolcross-over before and after the immersion in a methanol solution. The polymer electrolyte can be used stably in a solid polymer fuel cell during the long-term operation of the solid polymer fuel celland enables excellent start-up performance of the solid polymer fuel cell.