A polymer electrolyte fuel cell comprising: a membrane-electrode assembly including a polymer electrolyte membrane and a pair of electrodes (4A, 4B) sandwiching a portion of the polymer electrolyte membrane, the portion being located on an inner side of a peripheral portion of the polymer electrolyte membrane; an electrically-conductive first separator having a plate shape, disposed to contact one of the pair of electrodes (4A, 4B) of the membrane-electrode assembly, and formed such that a groove-like first reactant gas channel is formed on one main surface of the first separator so as to bend, said one main surface contacting said one of the pair of electrodes (4A, 4B); and an electrically-conductive second separator having a plate shape, disposed to contact the other one of the pair of electrodes (4A, 4B) of the membrane-electrode assembly, and formed such that a groove-like second reactant gas channel is formed on one main surface of the second separator so as to bend, said one main surface contacting said other one of the pair of electrodes (4A, 4B), wherein: the first reactant gas channel is formed such that when viewed in a thickness direction of the first separator, a portion (hereinafter referred to as a "first particular portion") of the first reactant gas channel is smaller in width than each of a portion located upstream of the first particular portion of the first reactant gas channel and a portion located downstream of the first particular portion of the first reactant gas channel, the first particular portion being within a region of said one of the pair of electrodes (4A, 4B), being located downstream of a portion where the first reactant gas channel extending from an upstream end thereof first overlaps the second reactant gas channel, and including a portion (hereinafter referred to as a "first portion") where the first reactant gas channel first separates from the second reactant gas channel; and the first particular portion is constituted by a portion extending to an upstream side of the first reactant gas channel from the first portion and a portion extending to a downstream side of the first reactant gas channel from the first portion. a membrane-electrode assembly including a polymer electrolyte membrane and a pair of electrodes (4A, 4B) sandwiching a portion of the polymer electrolyte membrane, the portion being located on an inner side of a peripheral portion of the polymer electrolyte membrane; an electrically-conductive first separator having a plate shape, disposed to contact one of the pair of electrodes (4A, 4B) of the membrane-electrode assembly, and formed such that a groove-like first reactant gas channel is formed on one main surface of the first separator so as to bend, said one main surface contacting said one of the pair of electrodes (4A, 4B); and an electrically-conductive second separator having a plate shape, disposed to contact the other one of the pair of electrodes (4A, 4B) of the membrane-electrode assembly, and formed such that a groove-like second reactant gas channel is formed on one main surface of the second separator so as to bend, said one main surface contacting said other one of the pair of electrodes (4A, 4B), the first reactant gas channel is formed such that when viewed in a thickness direction of the first separator, a portion (hereinafter referred to as a "first particular portion") of the first reactant gas channel is smaller in width than each of a portion located upstream of the first particular portion of the first reactant gas channel and a portion located downstream of the first particular portion of the first reactant gas channel, the first particular portion being within a region of said one of the pair of electrodes (4A, 4B), being located downstream of a portion where the first reactant gas channel extending from an upstream end thereof first overlaps the second reactant gas channel, and including a portion (hereinafter referred to as a "first portion") where the first reactant gas channel first separates from the second reactant gas channel; and the first particular portion is constituted by a portion extending to an upstream side of the first reactant gas channel from the first portion and a portion extending to a downstream side of the first reactant gas channel from the first portion.