NOVELTY - A fuel cell system comprises a secondary cell (200) that receives and stores surplus electric power by which output electric power of the fuel cell is greater than demanded electric power; and that compensates for shortfall by which the output electric power of the fuel cell is less than the demanded electric power demanded of the fuel cell system if the output electric power of the fuel cell is less than the demanded electric power demanded of the fuel cell system.    USE - The fuel cell system is used for converting the chemical energy from a fuel into electricity.    ADVANTAGE - The fuel cell system has improved durability.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell; a secondary cell (200) that receives and stores surplus electric power by which output electric power of the fuel cell is greater than demanded electric power which is demanded of the fuel cell system if the output electric power of the fuel cell is greater than the demanded electric power which is demanded of the fuel cell system, and that compensates for shortfall by which the output electric power of the fuel cell is less than the demanded electric power demanded of the fuel cell system if the output electric power of the fuel cell is less than the demanded electric power demanded of the fuel cell system; a voltage measurement portion that measures voltage of the fuel cell; a current measurement portion that measures electric current of the fuel cell; and a control portion (700) that performs a control such that the voltage of the fuel cell does not become greater than or equal to a pre-set high-potential avoidance voltage. If a current-voltage characteristic of the fuel cell declines to a level that is lower by at least a pre-determined amount than a level of the current-voltage characteristic of the fuel cell which occurs during an early period, the control portion executes at least one of (i) an operation of re-setting the high-potential avoidance voltage to a value that is smaller than an early-period set value and (ii) an operation of re-setting a remaining charge target value that is a lower-limit of amount of charge held by the second cell to a value that is larger than a set value used during the early period.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a fuel cell system.    Secondary cell (200)    Converter (500)    Cell controller (600)    Control portion (700)    Load (800)