Provided is a fuel cell, the output voltage of which is improved by making a membrane moist state uniform. An anode-side gas diffusion layer and a cathode-side gas diffusion layer are joined to a membrane electrode assembly, and a separator is joined to the anode-side gas diffusion layer. The separator has a recess portion and a protrusion portion formed to constitute a gas flow path and a refrigerant flow path, respectively. The cross-sectional area of the recess portion is made relatively small at the downstream side in comparison with that at the upstream side, and the cross-sectional area of the protrusion portion is made relatively large at the downstream side in comparison with that at the upstream side, thereby improving the moist state.