This invention relates to an ion-conducting binder used for a membrane electrode assembly for polymer electrolyte fuel cells, the assembly consisting of a polymer electrolyte membrane and two gas diffusion electrodes stuck to the polymer electrolyte membrane with the membrane put between the electrodes, which binder comprises a block copolymer which comprises a polymer block (A) having as a main unit an aromatic vinyl compound unit whose .alpha.-carbon is quaternary carbon, and a flexible polymer block (B), and has ion-conducting groups on the polymer block (A), and a solution or suspension thereof, and a membrane electrode assembly and a polymer electrolyte fuel cell.The ion-conducting binder, membrane electrode assembly and polymer electrolyte fuel cell of this invention are economical, mild to the environment and excellent in moldability and oxidation stability.