A fuel cell system comprising: a fuel cell generating an electric power from a fuel and an oxidant gas; a combusting portion generating an exhaust combustion gas by burning an anode off-gas from the fuel cell and a cathode off-gas from the fuel cell; an oxidant gas supply device (11 c1) supplying the oxidant gas to the combusting portion; a casing housing the fuel cell and the combusting portion; a fuel cell chamber installed in an indoor environment and at least housing the casing and the oxidant gas supply device (11c1), the fuel cell chamber including an airtight structure; a ventilation exhaust line including an intake portion that suctions air from an outdoor environment to send the air into the fuel cell chamber and an exhaust portion that discharges air within the fuel cell chamber into the outdoor environment; a combustion exhaust line including a cathode air supply pipe that connects an inside of the fuel cell chamber and the fuel cell each other to supply air including the oxidant gas within the fuel cell chamber to the fuel cell and an exhaust pipe that connects the combusting portion and an outside of the fuel cell chamber each other to discharge the exhaust combustion gas generated at the combusting portion into the outdoor environment; a first fan provided at the intake portion of the ventilation exhaust line to suction air into the fuel cell chamber; a second fan provided at the exhaust portion of the ventilation exhaust line to discharge air from the fuel cell chamber; and a control unit controlling the first fan and the second fan, the fuel cell system further comprising an evaporating portion that generates a water vapor by evaporating water for reforming, wherein the control unit drives at least the second fan during a power generation operation of the fuel cell system to maintain the inside of the fuel cell chamber at a negative pressure, wherein the control unit controls the second fan to be stopped and drives the first fan to maintain the inside of the fuel cell chamber at a positive pressure until a generation of water vapour within the casing is stopped after a stop operation of the fuel cell system is started. a fuel cell generating an electric power from a fuel and an oxidant gas; a combusting portion generating an exhaust combustion gas by burning an anode off-gas from the fuel cell and a cathode off-gas from the fuel cell; an oxidant gas supply device (11 c1) supplying the oxidant gas to the combusting portion; a casing housing the fuel cell and the combusting portion; a fuel cell chamber installed in an indoor environment and at least housing the casing and the oxidant gas supply device (11c1), the fuel cell chamber including an airtight structure; a ventilation exhaust line including an intake portion that suctions air from an outdoor environment to send the air into the fuel cell chamber and an exhaust portion that discharges air within the fuel cell chamber into the outdoor environment; a combustion exhaust line including a cathode air supply pipe that connects an inside of the fuel cell chamber and the fuel cell each other to supply air including the oxidant gas within the fuel cell chamber to the fuel cell and an exhaust pipe that connects the combusting portion and an outside of the fuel cell chamber each other to discharge the exhaust combustion gas generated at the combusting portion into the outdoor environment; a first fan provided at the intake portion of the ventilation exhaust line to suction air into the fuel cell chamber; a second fan provided at the exhaust portion of the ventilation exhaust line to discharge air from the fuel cell chamber; and a control unit controlling the first fan and the second fan, the fuel cell system further comprising an evaporating portion that generates a water vapor by evaporating water for reforming, wherein the control unit drives at least the second fan during a power generation operation of the fuel cell system to maintain the inside of the fuel cell chamber at a negative pressure,