1. A fuel cell system comprising: a fuel cell that receives supply of reactant gas to generate power; a secondary battery for storing part of the power generated by the fuel cell; calculation means which, when surplus power for high potential avoidance is added to output of the fuel cell to avoid a high potential, calculates high potential avoidance voltage to avoid the high potential by feedback control based on a deviation between power generation voltage of the fuel cell and high potential avoidance target voltage; determination means for determining whether an operating state of the fuel cell corresponds to a prohibition condition to prohibit updating of an integral term of the feedback control or a permission condition; and feedback control condition deciding means for stopping the updating of the integral term when the operating state corresponds to the prohibition condition or updating the integral term when the operating state corresponds to the permission condition.