A fuel cell (1) is constituted by stacking membrane electrode assemblies (2) and separators (3), wherein fuel gas passages (16), oxidizing gas passages (17) and coolant passages (9), which are formed by the separators (3), are arranged in parallel along the outer surfaces of gas diffusion layers (6A) and (6C) of the membrane electrode assemblies (2). With such a constitution, a thickness dimension of the separators (3) can be reduced, and with respect to a stacking direction of the fuel cell stack, the number of stacked unit cells can be increased in the same dimensions as that of the conventional fuel cell stack. Therefore, a fuel cell stack with a high output density can be obtained.