A battery voltage Ef of a fuel cell 1 is detected, and a current instruction value which transformed an electric power instruction value Pfc of the fuel cell is reduced at falling-down of the voltage Ef. A current limiter 8 makes the current instruction value a limit value at the time of the falling-down of Ef, and reduces the limit value according to the fall of Ef. For example, current instruction value is restricted so as to begin to reduce battery current IFC at voltage drop alarm level (first threshold value), and become zero at voltage drop protection level (second threshold value).