In a membrane-electrode assembly for polymer electrolyte fuel cells comprising a polymer electrolyte membrane and two gas diffusion electrodes being bonded to the membrane so that the membrane can be between them, at least one catalyst layer constituting the gas diffusion electrodes characterized in that the ion-conductive binder comprises a block copolymer having a particle size of 1 μm or less comprising a polymer block (A) having ion-conductive groups and a polymer block (B) having no ion-conductive group, both polymer blocks phase separate from each other, polymer block (A) forms a continuous phase, and the contact parts of the block copolymer with catalyst particles are comprised of polymer block (A) having ion-conductive groups; a membrane-electrode assembly and a polymer electrolyte fuel cell.