A membrane-electrode assembly (1) of the present invention comprises: a quadrate polymer electrolyte membrane (2); a pair of catalyst layers provided to sandwich the polymer electrolyte membrane except for a peripheral portion of the polymer electrolyte membrane; and a pair of gas diffusion layers (3) provided respectively on the pair of the catalyst layers, the membrane-electrode assembly (1) being incorporated into a fuel cell by being sandwiched between a pair of separators on each of which a reaction gas passage (A) or (C) is concavely formed in a gas diffusion layer contacting region of an inner surface thereof, the gas diffusion layer contacting region being a region contacting the gas diffusion layer, wherein: each of the reaction gas passages (A) and (C) in the gas diffusion layer contacting region is formed to have a serpentine shape which extends from upstream to downstream in a direction from a first side (2a) of the polymer electrolyte membrane 1 to a third side (2c) facing the first side along a second side (2b) adjacent to the first side while turning in directions along the first side; reinforced portions (4) for reinforcing the polymer electrolyte membrane are formed at a portion corresponding to the second side and a portion corresponding to a fourth side (2d) facing the second side in the peripheral portion of the polymer electrolyte membrane 2; and the reinforced portion (4) is not formed at a portion corresponding to at least the third side (2c) in the peripheral portion of the polymer electrolyte membrane (2).