A polymer electrolyte fuel cell comprising: a membrane-electrode assembly including a polymer electrolyte membrane and a pair of electrodes (4A, 4B) sandwiching the polymer electrolyte membrane; and a pair of separators (6A, 6B) each formed in a plate shape and having electrical conductivity, wherein: each of the separators (6A, 6B) includes a peripheral portion (16A, 16B) and a portion (26A, 26B) other than the peripheral portion; characterized in that when viewed from a thickness direction of the separator (6A, 6B), the peripheral portion (16A, 16B) of the separator is formed in a completely annular shape and is a region located on an inner side of an outer periphery of the separator; and the separator (6A, 6B) is configured such that the porosity of the peripheral portion (16A, 16B) is higher than that of the portion (26A, 26B) other than the peripheral portion, wherein: each of the electrodes (4A, 4B) includes a catalyst layer (2A, 2B) and a gas diffusion layer (3A, 3B); groove-shaped reactant gas channels and rib portions (11, 12) each located between the groove-shaped reactant gas channels are formed on at least one of one main surface, contacting the gas diffusion layer (3A, 3B), of the separator and one main surface, contacting the separator, of the gas diffusion layer; a groove-shaped water channel is formed on the other main surface of at least one of the pair of separators (6A, 6B); and the water channel is formed such that when viewed from the thickness direction of the separator (6A, 6B), a part of the water channel overlaps the peripheral portion of the separator. a membrane-electrode assembly including a polymer electrolyte membrane and a pair of electrodes (4A, 4B) sandwiching the polymer electrolyte membrane; and a pair of separators (6A, 6B) each formed in a plate shape and having electrical conductivity, wherein: each of the separators (6A, 6B) includes a peripheral portion (16A, 16B) and a portion (26A, 26B) other than the peripheral portion; characterized in that when viewed from a thickness direction of the separator (6A, 6B), the peripheral portion (16A, 16B) of the separator is formed in a completely annular shape and is a region located on an inner side of an outer periphery of the separator; and the separator (6A, 6B) is configured such that the porosity of the peripheral portion (16A, 16B) is higher than that of the portion (26A, 26B) other than the peripheral portion, wherein: each of the electrodes (4A, 4B) includes a catalyst layer (2A, 2B) and a gas diffusion layer (3A, 3B); groove-shaped reactant gas channels and rib portions (11, 12) each located between the groove-shaped reactant gas channels are formed on at least one of one main surface, contacting the gas diffusion layer (3A, 3B), of the separator and one main surface, contacting the separator, of the gas diffusion layer; a groove-shaped water channel is formed on the other main surface of at least one of the pair of separators (6A, 6B); and the water channel is formed such that when viewed from the thickness direction of the separator (6A, 6B), a part of the water channel overlaps the peripheral portion of the separator. each of the separators (6A, 6B) includes a peripheral portion (16A, 16B) and a portion (26A, 26B) other than the peripheral portion; characterized in that when viewed from a thickness direction of the separator (6A, 6B), the peripheral portion (16A, 16B) of the separator is formed in a completely annular shape and is a region located on an inner side of an outer periphery of the separator; and the separator (6A, 6B) is configured such that the porosity of the peripheral portion (16A, 16B) is higher than that of the portion (26A, 26B) other than the peripheral portion, wherein: each of the electrodes (4A, 4B) includes a catalyst layer (2A, 2B) and a gas diffusion layer (3A, 3B); groove-shaped reactant gas channels and rib portions (11, 12) each located between the groove-shaped reactant gas channels are formed on at least one of one main surface, contacting the gas diffusion layer (3A, 3B), of the separator and one main surface, contacting the separator, of the gas diffusion layer; a groove-shaped water channel is formed on the other main surface of at least one of the pair of separators (6A, 6B); and the water channel is formed such that when viewed from the thickness direction of the separator (6A, 6B), a part of the water channel overlaps the peripheral portion of the separator. each of the electrodes (4A, 4B) includes a catalyst layer (2A, 2B) and a gas diffusion layer (3A, 3B); groove-shaped reactant gas channels and rib portions (11, 12) each located between the groove-shaped reactant gas channels are formed on at least one of one main surface, contacting the gas diffusion layer (3A, 3B), of the separator and one main surface, contacting the separator, of the gas diffusion layer; a groove-shaped water channel is formed on the other main surface of at least one of the pair of separators (6A, 6B); and the water channel is formed such that when viewed from the thickness direction of the separator (6A, 6B), a part of the water channel overlaps the peripheral portion of the separator.