A fuel cell stack comprises membrane electrode assemblies (3) in which gas diffusion electrodes (2a,2b) are arranged on both sides of an ion exchange membrane (1) and a reactant gas supply separators (5) interposed between the membrane electrode assemblies (3). The reactant gas supply separators (5) each has a first surface having first reactant gas supply grooves (9a) for supplying first reactant gas, a second surface having second reactant gas supply grooves (9b) for supplying an second reactant gas, and water supply means for supplying water to the first reactant gas supply grooves (9a). The water supply means has a water supply grooves (15) for introducing water from a water supply manifold (14) disposed on the second surface, communication holes (16) for communication of the second surface with the first surface, and a buffer section (17) having a porous body (20) for uniformly distributing water from the water supply grooves (15) to the communication holes (16). The porous body (20) extends along the second surface. The water flows in the porous body (20) along the second surface. Thus, water can be supplied uniformly to the first reactant gas supply grooves (9a) of each of the reactant gas supply separators (5).