A fuel cell system comprising a stack including at least one electricity generator for generating electric energy through a reaction between fuel and oxygen, a fuel supply unit for supplying the fuel to the electricity generator, and an oxygen supply unit for supplying the oxygen to the electricity generator is provided. The electricity generator includes a membrane-electrode assembly and separators which have a plurality of flow channels for passing fuel and oxygen and which are placed on both surfaces of the membrane-electrode assembly. The separators include a connecting portion with a straight part between rounded corners for coupling the flow channels at their ends. The connecting portion minimizes turbulence in the flow while maintaining sufficient effective area for contact between the fuel and oxygen with the active area of the membrane-electrode assembly.