A polymer electrolyte fuel cell of the present invention includes a membrane electrode assembly (5) having a pair of electrodes (4a, 4b) sandwiching a portion of a polymer electrolyte membrane (1) which is inward relative to a peripheral portion thereof, a first separator (6a), and a second separator (6b), the first separator (6a) is provided with a first reaction gas channel (8) on one main surface, the second separator (6b) is provided with a second reaction gas channel (9) on one main surface such that the second reaction gas channel (9) has a second rib portion (12), the first reaction gas channel (8) is formed such that a ratio of a first reaction gas channel width of an upstream portion (18b) to the second rib portion (12) is set larger than a ratio of a first reaction gas channel width of a downstream portion (18c) to the second rib portion (12), and the ratio of the first reaction gas channel width of the upstream portion (18b) to the second rib portion (12) is a predetermined ratio.