A fuel cell system includes: a fuel cell stack including a plurality of fuel cells connected in series; a fuel supply portion supplying a fuel to each fuel cell; a current regulation portion regulating an electric current flowing from the fuel cell stack in such a way that an output voltage of the fuel cell stack is a predetermined set voltage; a voltage detection portion detecting an output voltage of each fuel cell; a fuel-supply control portion regulating a fuel supply to each fuel cell by the fuel supply portion in such a way that the difference decreases between an output voltage of each fuel cell detected by the voltage detection portion; a fuel-stoichiometric ratio detection portion detecting a fuel stoichiometric ratio of the fuel cell stack; and a fuel-stoichiometric ratio control portion regulating the set voltage in such a way that a fuel stoichiometric ratio detected by the fuel-stoichiometric ratio detection portion is a target fuel stoichiometric ratio set in advance.