athode 5 and anode 6 of the PEFC 1, that charges therein a portion of the output of the PEFC 1 when a power demand is smaller than the output of the PEFC 1, that discharges it when the power demand is larger than the output of the PEFC 1, and that is preferably electric double layer capacitor 7; a secondary battery, such as a lithium ion battery, that is connected in parallel with the cathode 5 an