Polymer electrolyte fuel cell and fuel cell stack comprising the same

A polymer electrolyte fuel cell of the present invention comprises a membrane-electrode assembly (5), a first separator (6a), and a second separator (6b); the first separator (6a) having a groove-shaped first reaction gas channel (8) on one main surface of the first separator (6a) which contacts the first electrode (4a) such that a plurality of straight-line-shaped first rib portions (11) run along each other; the second electrode (4b) having a groove-shaped second reaction gas channel (9) on one main surface of the second electrode (4b) which contacts the second separator (6b) such that a plurality of straight-line-shaped second rib portions (12) run along each other; a ratio of a first reaction gas channel width of at least an upstream portion (18b) of the first reaction gas channel (8) with respect to a second rib portion (12) is greater than 0 and not greater than 1.