A polymer electrolyte fuel cell includes: a membrane-electrode assembly (10) having a polymer electrolyte membrane (1) and a pair of electrodes (4, 8) sandwiching a portion of the polymer electrolyte membrane (1) which portion is located inwardly of a peripheral portion of the polymer electrolyte membrane (1); an electrically-conductive first separator (30) disposed to contact the membrane-electrode assembly (10) and formed such that a groove-like first reactant gas channel (37) is formed on one main surface thereof so as to bend; and an electrically-conductive second separator (20) disposed to contact the membrane-electrode assembly (10) and formed such that a groove-like second reactant gas channel (27) is formed on one main surface thereof so as to bend, wherein the first reactant gas channel (27) is formed such that a width of a portion of the first reactant gas channel (27) which portion is formed at least a portion (hereinafter referred to as an uppermost stream portion 8C of the first separator 30) located between a portion where the first reactant gas channel (27) extending from an upstream end thereof first contacts the electrode 8 and a portion where the second reactant gas channel (27) extending from an upstream end thereof first contacts the electrode 4 is smaller than a width of a portion of the first reactant gas channel (27) which portion is formed at a portion other than the uppermost stream portion 8C of the first separator 30.