The amount of fuel supplied to a fuel cell is set to a second set value Qm2 smaller than a first set value Qm1 determined based on a load. Then, an output current Ifcr of the fuel cell with the fuel supply amount set to the second set value Qm2 is detected. The output current Ifcr is compared with a reference value Iref for determining mild deterioration to determine whether the fuel cell has deteriorated from the comparison result. If a determination that the fuel cell has deteriorated is made, the fuel supply amount is reset to a third set value Qm3 larger than the second set value Qm2.