A gas channel forming plate is arranged between a membrane electrode assembly and a flat separator base. The gas channel forming plate includes gas channels arranged on a surface that faces the membrane electrode assembly, water channels each formed on the back side of the protrusion between an adjacent pair of the gas channels, communication passages that connect the gas channels and the water channels to each other, and guide portions formed by causing an inner wall surface of a gas channel to protrude inward in the gas channel. The guide portions are formed such that the upstream edge of each communication passage is arranged in a range in which, in the velocity vector of the gas flowing in the gas channel, the directional component directed from the side corresponding to the membrane electrode assembly toward the flat separator base has a positive value.