A fuel cell system includes: a fuel cell; a secondary cell that receives and stores surplus power by which output of the fuel cell is greater than power demanded of the system if the output is so, and that compensates for shortfall by which the output of the fuel cell is less than the power demanded of the system if the output is so; a voltage measurement portion that measures voltage of the fuel cell; a current measurement portion that measures current of the fuel cell; and a control portion that performs a control such that the voltage of the fuel cell does not exceed or equal a pre-set high-potential avoidance voltage. If a current-voltage characteristic of the fuel cell declines by at least a pre-determined amount from an early-period level, the control portion re-sets the high-potential avoidance voltage to a value that is smaller than an early-period set value.