A gas channel forming member is located between a separator and a membrane electrode assembly. The gas channel forming member includes gas channels, which are arranged in parallel with each other on a surface that faces the membrane electrode assembly, water channels, which are formed on a surface that faces the separator, and inner communication passages. The water channels are each located between adjacent two of the gas channels. The inner communication passages communicate the gas channels and the water channels with each other. Each water channel is formed such that the flow cross-sectional area in an outlet section including an outlet opening is larger than the cross-sectional area of an upstream section, which is upstream of an adjacent to the outlet section in the flow direction of the oxidant gas.