In a fuel cell stack, gas channels and heat medium channels are disposed on one surface and the other surface of one plate, respectively. Gas channels are disposed on the other plate such that they face the gas channels in the one plate. A gas inlet header is disposed at the upper part of the gas channel in the plate and a heat medium inlet header is disposed at the upper part of the heat medium channels such that they face the gas inlet header on the other side. Cooling water as a heat medium is supplied from a heat medium supply manifold hole to a heat medium inlet header. Water vapor in the reaction gas (wet fuel gas) is prevented from being condensed in the inlet area of the gas channels by heating the gas inlet header by heat conduction.