NOVELTY - The method involves detecting hydrogen peroxide concentration during power generation by a hydrogen peroxide concentration sensor (60) provided directly on a membrane electrode assembly (24). An operating condition of a fuel cell (10) is determined based on the detected concentration. A map is created based on a relationship between the concentration and fuel cell voltage to be corrected in correspondence with shape of gas flow field of the fuel cell or characteristic of the assembly, where control of the fuel cell is implemented based on the fuel cell voltage obtained from the map.    USE - Method for operating a fuel cell (claimed) i.e. solid polymer electrolyte fuel cell, formed by stacking a membrane electrode assembly and a separator such as bipolar plate, in an electric vehicle.    ADVANTAGE - The method enables providing the hydrogen peroxide concentration sensor directly on the membrane electrode assembly so as to promptly and reliably detect concentration of hydrogen peroxide generated in the membrane electrode assembly, thus reliably detecting the hydrogen peroxide concentration in real time, and hence effectively suppressing degradation of an electrolyte membrane, and controlling operation of the fuel cell in a suitable state. The method enables allowing flow rate of oxygen-containing gas supplied to a cathode to be increased if decrease in cathode electric potential and increase in hydrogen peroxide concentration are detected, thus effectively reducing the hydrogen peroxide concentration and reliably adjusting the concentration to a desired value. The method enables decreasing temperature of a coolant to cause elution of hydrogen peroxide from the solid polymer electrolyte membrane to perform operation under conditions where the control of condensed water is implemented, so that power generation stability can be achieved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a fuel cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a fuel cell system.    Fuel cell (10)    Membrane electrode assembly (24)    Hydrogen peroxide concentration sensor (60)    Air compressor (70)    Humidifier (74)