POLYELECTROLYTE FUEL CELL, FUEL CELL STACK PROVIDED WITH SAME, FUEL CELL SYSTEM, AND OPERATION METHOD FOR FUEL CELL SYSTEM

A polymer electrolyte fuel cell of the present invention includes: a membrane-electrode assembly (5) having a polymer electrolyte membrane (1) and a pair of electrodes (4A and 4B); a first separator (6A) having one main surface on which a groove-like first reactant gas channel (8) is formed so as to bend; and a second separator (6B) having one main surface on which a groove-like second reactant gas channel (9) is formed so as to bend. When viewed in a thickness direction of the first separator (6A), the first reactant gas channel (8) is formed such that a first particular portion (51) of the first reactant gas channel (8) is smaller in width than each of a portion located upstream of the first particular portion(51) and a portion located downstream of the first particular portion(51), the first particular portion (51) being within a region of the electrode (4A) and including a portion where the first reactant gas channel (8) extending from an upstream end thereof first separates from the second reactant gas channel (9).