A fuel cell system, comprising: a fuel cell having a fuel electrode to which a fuel gas is supplied and an oxidant electrode to which an oxidant gas is supplied, the fuel cell configured to generate electricity via a reaction between the supplied fuel gas and the oxidant gas; a gas discharge unit for discharging a discharge gas from the fuel electrode of the fuel cell; an external load (110, 120) for receiving power from the fuel cell; and a controller unit operable to: enter an idle stop mode if a driving power does not need to be supplied to the external load (110, 120) from the fuel cell, wherein the fuel gas and the oxidant gas are not supplied to the fuel cell and electricity is not generated by the fuel cell during the idle stop mode; and end the idle stop mode if the driving power needs to be supplied to the external load (110, 120) from the fuel cell, wherein the supply of the fuel gas and the oxidant gas is restarted and electricity generation by the fuel cell resumes when the idle stop mode ends; characterized in that the controller unit is further operable to: begin an idle stop prohibition time period that starts when the idle mode ends, the idle stop prohibition time period lasting for a predetermined time; prohibit entry into the idle stop mode during the idle stop prohibition time period even if the driving power no longer needs to be supplied to the external load (110, 120) from the fuel cell, such that the supply of the fuel gas and the oxidant gas is supplied to the fuel cell from the time that the idle stop mode ends at least until the idle stop prohibition time period ends; and discharge the discharge gas from the fuel cell via the gas discharge unit from the time that the idle stop mode ends at least until the idle stop prohibition time period ends. a fuel cell having a fuel electrode to which a fuel gas is supplied and an oxidant electrode to which an oxidant gas is supplied, the fuel cell configured to generate electricity via a reaction between the supplied fuel gas and the oxidant gas; a gas discharge unit for discharging a discharge gas from the fuel electrode of the fuel cell; an external load (110, 120) for receiving power from the fuel cell; and a controller unit operable to: enter an idle stop mode if a driving power does not need to be supplied to the external load (110, 120) from the fuel cell, wherein the fuel gas and the oxidant gas are not supplied to the fuel cell and electricity is not generated by the fuel cell during the idle stop mode; and end the idle stop mode if the driving power needs to be supplied to the external load (110, 120) from the fuel cell, wherein the supply of the fuel gas and the oxidant gas is restarted and electricity generation by the fuel cell resumes when the idle stop mode ends; enter an idle stop mode if a driving power does not need to be supplied to the external load (110, 120) from the fuel cell, wherein the fuel gas and the oxidant gas are not supplied to the fuel cell and electricity is not generated by the fuel cell during the idle stop mode; and end the idle stop mode if the driving power needs to be supplied to the external load (110, 120) from the fuel cell, wherein the supply of the fuel gas and the oxidant gas is restarted and electricity generation by the fuel cell resumes when the idle stop mode ends; characterized in that the controller unit is further operable to: begin an idle stop prohibition time period that starts when the idle mode ends, the idle stop prohibition time period lasting for a predetermined time; prohibit entry into the idle stop mode during the idle stop prohibition time period even if the driving power no longer needs to be supplied to the external load (110, 120) from the fuel cell, such that the supply of the fuel gas and the oxidant gas is supplied to the fuel cell from the time that the idle stop mode ends at least until the idle stop prohibition time period ends; and discharge the discharge gas from the fuel cell via the gas discharge unit from the time that the idle stop mode ends at least until the idle stop prohibition time period ends. begin an idle stop prohibition time period that starts when the idle mode ends, the idle stop prohibition time period lasting for a predetermined time; prohibit entry into the idle stop mode during the idle stop prohibition time period even if the driving power no longer needs to be supplied to the external load (110, 120) from the fuel cell, such that the supply of the fuel gas and the oxidant gas is supplied to the fuel cell from the time that the idle stop mode ends at least until the idle stop prohibition time period ends; and discharge the discharge gas from the fuel cell via the gas discharge unit from the time that the idle stop mode ends at least until the idle stop prohibition time period ends.