A polymer electrolyte fuel cell comprising: a membrane-electrode assembly including a polymer electrolyte membrane and first and second electrodes (4a, 4b) sandwiching a pair of main surfaces of the polymer electrolyte membrane; an electrically-conductive first separator which is of a plate shape and is provided in contact with the first electrode of the membrane-electrode assembly; and an electrically-conductive second separator which is of a plate shape and is provided in contact with the second electrode of the membrane-electrode assembly, the first electrode or the first separator being provided with a groove-shaped first reaction gas channel on one main surface of the first electrode /the first separator which contacts the first separator /the first electrode such that a plurality of first rib portions of a straight-line shape run along each other; the second electrode being provided with a groove-shaped second reaction gas channel on one main surface of the second electrode which contacts the second separator such that a plurality of second rib portions (12, 12a, 12b) of a straight-line shape run along each other; wherein an upstream portion of the first reaction gas channel is defined as a portion extending over a predetermined length from a portion which contacts the first electrode first from an upstream end of the first reaction gas channel toward a downstream side; a downstream portion of the first reaction gas channel is defined as a portion of the first reaction gas channel which is downstream of the upstream portion of the first reaction gas channel; and when a ratio of a first reaction gas channel width to the second rib portion (12, 12a, 12b) is defined as a ratio of a portion of the first reaction gas channel which overlaps with the second rib portion (12, 12a, 12b) in a width direction of the first reaction gas channel with respect to an overall width of the first reaction gas channel when viewed in a thickness direction of the first separator, characterized in that a ratio of a first reaction gas channel width of the upstream portion of the first reaction gas channel with respect to the second rib portion (12, 12a, 12b) is greater than 0 and not greater than 1, and wherein the ratio of the first reaction gas channel width of the upstream portion of the first reaction gas channel with respect to the second rib portion (12, 12a, 12b) is greater than a ratio of a first reaction gas channel width of the downstream portion of the first reaction gas channel with respect to the second rib portion (12, 12a, 12b). a membrane-electrode assembly including a polymer electrolyte membrane and first and second electrodes (4a, 4b) sandwiching a pair of main surfaces of the polymer electrolyte membrane; an electrically-conductive first separator which is of a plate shape and is provided in contact with the first electrode of the membrane-electrode assembly; and an electrically-conductive second separator which is of a plate shape and is provided in contact with the second electrode of the membrane-electrode assembly, the first electrode or the first separator being provided with a groove-shaped first reaction gas channel on one main surface of the first electrode /the first separator which contacts the first separator /the first electrode such that a plurality of first rib portions of a straight-line shape run along each other; the second electrode being provided with a groove-shaped second reaction gas channel on one main surface of the second electrode which contacts the second separator such that a plurality of second rib portions (12, 12a, 12b) of a straight-line shape run along each other; wherein an upstream portion of the first reaction gas channel is defined as a portion extending over a predetermined length from a portion which contacts the first electrode first from an upstream end of the first reaction gas channel toward a downstream side; a downstream portion of the first reaction gas channel is defined as a portion of the first reaction gas channel which is downstream of the upstream portion of the first reaction gas channel; and when a ratio of a first reaction gas channel width to the second rib portion (12, 12a, 12b) is defined as a ratio of a portion of the first reaction gas channel which overlaps with the second rib portion (12, 12a, 12b) in a width direction of the first reaction gas channel with respect to an overall width of the first reaction gas channel when viewed in a thickness direction of the first separator, characterized in that a ratio of a first reaction gas channel width of the upstream portion of the first reaction gas channel with respect to the second rib portion (12, 12a, 12b) is greater than 0 and not greater than 1, and wherein the ratio of the first reaction gas channel width of the upstream portion of the first reaction gas channel with respect to the second rib portion (12, 12a, 12b) is greater than a ratio of a first reaction gas channel width of the downstream portion of the first reaction gas channel with respect to the second rib portion (12, 12a, 12b).