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A fuel cell has a fuel gas passage disposed therein, a circulation passage connecting a fuel gas pump in series to the fuel gas passage, and an intermediate fuel gas supply mechanism. The intermediate fuel gas supply mechanism supplies a fuel gas, which is lower in humidity than a fuel gas flowing u

A fuel cell has a fuel gas passage disposed therein, a circulation passage connecting a fuel gas pump in series to the fuel gas passage, and an intermediate fuel gas supply mechanism. The intermediate fuel gas supply mechanism supplies a fuel gas, which is lower in humidity than a fuel gas flowing upstream of a intermediate fuel gas supply port in the fuel gas passage, to the intermediate fuel gas supply port, for thereby preventing water from being condensed in the fuel gas passage.

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1. A fuel cell comprising:a membrane electrode assembly comprising a solid polymer ion exchange membrane, an anode electrode mounted on one surface of said solid polymer ion exchange membrane, and a cathode electrode mounted on the other surface of said solid polymer ion exchange membrane: an anode

1. A fuel cell comprising:a membrane electrode assembly comprising a solid polymer ion exchange membrane, an anode electrode mounted on one surface of said solid polymer ion exchange membrane, and a cathode electrode mounted on the other surface of said solid polymer ion exchange membrane: an anode separator; a cathode separator, said membrane electrode assembly being sandwiched between said anode separator and said cathode separator; a fuel gas passage disposed between said anode electrode and said anode separator; a circulation passage connecting a fuel gas pump disposed outside of the fuel cell in series to said fuel gas passage; an intermediate fuel gas supply port connected to said fuel gas passage between a fuel gas inlet and a fuel gas outlet thereof; and an intermediate fuel gas supply mechanism for supplying a fuel gas, which is lower in humidity than a fuel gas flowing upstream of said intermediate fuel gas supply port, from said intermediate fuel gas supply port to said fuel gas passage. 2. A fuel cell according to claim 1, wherein said fuel gas passage comprises a meandering passage having a curved end, said intermediate fuel gas supply port being connected to said curved end.3. A fuel cell according to claim 2, further comprising:a single intermediate fuel gas passage connected to said intermediate fuel gas supply port through an orifice. 4. A fuel cell according to claim 1, wherein said intermediate fuel gas supply mechanism has a temperature regulator for regulating the temperature of the fuel gas supplied to said intermediate fuel gas supply port so as to be substantially equal to the temperature of the fuel gas supplied through said circulation passage to said fuel gas passage.5. A fuel cell according to claim 4, wherein said temperature regulator comprises:a heat exchanger for effecting a heat exchange between a cooling medium which is discharged from said fuel cell after having cooled the fuel cell and the fuel gas supplied to said intermediate fuel gas supply port. 6. A fuel cell according to claim 4, wherein said intermediate fuel gas supply mechanism has an intermediate fuel gas passage connected to said circulation passage between a fuel gas discharge port of said fuel cell and a fuel gas inlet port of said fuel gas pump, for supplying a fuel gas, which is lower in humidity than and substantially identical in temperature to the fuel gas discharged from said fuel gas discharge port, to said circulation passage.7. A fuel cell according to claim 6, further comprising:a dew-point hygrometer connected to said circulation passage between said fuel gas inlet port of said fuel gas pump and the junction between said circulation passage and said single intermediate fuel gas passage. 8. A fuel cell according to claim 5, further comprising:a cooling medium supply port for supplying the cooling medium to said fuel cell; a cooling medium discharge port for discharging the cooling medium from said fuel cell; a fuel gas supply port for supplying the fuel gas to said fuel cell; a fuel gas discharge port for discharging the fuel gas from said fuel cell; and at least either one of a temperature sensor, a pressure sensor, and a flow sensor which is connected to each of said cooling medium supply port, said cooling medium discharge port, said fuel gas supply port, and said fuel gas discharge port. 9. A method of operating a fuel cell having a membrane electrode assembly comprising a solid polymer ion exchange membrane, an anode electrode mounted on one surface of said solid polymer ion exchange membrane, and a cathode electrode mounted on the other surface of said solid polymer ion exchange membrane, an anode separator, a cathode separator, said membrane electrode assembly being sandwiched between said anode separator and said cathode separator, a fuel gas passage disposed between said anode electrode and said anode separator, and an intermediate fuel gas supply port connected to said fuel gas passage between a fuel gas inlet and a fuel gas outlet thereof, said method comprising the step of:supplying a fuel gas, which is lower in humidity than a fuel gas flowing upstream of said intermediate fuel gas supply port, from said intermediate fuel gas supply port to said fuel gas passage, thereby preventing water from being condensed in said fuel gas passage. 10. A method according to claim 9, further comprising the step of:regulating the temperature of the fuel gas supplied to said intermediate fuel gas supply port so as to be substantially equal to the temperature of the fuel gas supplied to said fuel gas passage. 11. A method according to claim 9, further comprising the steps of:supplying a fuel gas circulatingly through a circulation passage connecting a fuel gas pump disposed outside of the fuel cell in series to said fuel gas passage; and supplying a fuel gas, which is lower in humidity than and substantially identical in temperature to the fuel gas discharged from a fuel gas discharge port of the fuel cell, to an intermediate fuel gas passage connected to said fuel gas discharge port and a fuel gas inlet port of said fuel gas pump. 12. A method according to claim 11, further comprising the step of:regulating the flow rate of the fuel gas supplied from said intermediate fuel gas passage with a dew-point hygrometer which is connected to said circulation passage between said fuel gas inlet port of said fuel gas pump and the junction between said circulation passage and said single intermediate fuel gas passage. 13. A method according to claim 10, wherein said fuel cell has a cooling medium supply port for supplying the cooling medium to said fuel cell, a cooling medium discharge port for discharging the cooling medium from said fuel cell, a fuel gas supply port for supplying the fuel gas to said fuel cell, a fuel gas discharge port for discharging the fuel gas from said fuel cell, and at least either one of a temperature sensor, a pressure sensor, and a flow sensor which is connected to each of said cooling medium supply port, said cooling medium discharge port, said fuel gas supply port, and said fuel gas discharge port, further comprising the steps of:detecting a change in the amount of heat with at least said temperature sensor, said pressure sensor, or said flow sensor; and regulating an operating pressure of said fuel cell based on the detected change in the amount of heat.