An improved fuel cell makes uniform partial gas pressure of a fuel gas flowing through a gas passage formed between an electrode and a separator of a fuel cell, activating an electrode reaction in a whole region of the electrode surface along the gas passage. The inlet and outlet for the fuel gas are disposed on a diagonal line of the separator to define a gas passage such that fuel gas smoothly flows even though portions of the passage away from the diagonal line. This may reduce the separator size and improve the volumetric efficiency of the fuel cell and the diffusibility of fuel gas as well as the drainage of water produced by the electrode reaction.