OPERATING METHOD OF FUEL CELL AND POWER SUPPLY

Disclosed are a method of operating a fuel cell and a power supply system, which are capable of suppressing condensation of moisture obstructing an air passage without the need of any complicated system configuration. The method of operating a fuel cell so as to output an electromotive force from the fuel cell by supplying air and a fuel gas thereto is characterized by including the step of controlling a supply of air while monitoring a relative humidity of exhausted air. To be more specific, the step of controlling the supply of air is performed such that the relative humidity of exhausted air becomes equal to or less than a specific humidity. The power supply system including, as a power supply, a fuel cell having an air supply unit, an air exhaust unit, and a fuel gas supply unit is characterized by including a humidity sensor provided on the air exhaust unit, and a control mechanism for controlling a supplied amount of air in the air supply unit on the basis of information given from the humidity sensor.