A combination of a fuel cell system and a combustion device, the fuel cell system comprising: a fuel cell which generates power by fuel and an oxidant gas; a combustion unit which generates a combustion exhaust 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 (11c1) which supplies the oxidant gas to the fuel cell; a housing which accommodates the fuel cell and the combustion unit; an air-tight fuel cell chamber which is provided in an indoor place so as to accommodate at least the housing and the oxidant gas supply device (11c1); a ventilation exhaust line which includes an intake unit introducing air from an outdoor place so as to suction the air into the fuel cell chamber and an exhaust unit discharging the air inside the fuel cell chamber so as to discharge the air into the outdoor place; a combustion exhaust line which includes a cathode air supply pipe causing the inside of the fuel cell chamber and the fuel cell to communicate with each other so that air including the oxidant gas inside the fuel cell chamber is supplied to the fuel cell and an exhaust pipe causing the combustion unit and the outside of the fuel cell chamber to communicate with each other so that a combustion exhaust gas generated in the combustion unit is discharged to the outdoor place; a first fan which is provided at the exhaust unit of the ventilation exhaust line so as to discharge air from the fuel cell chamber; a check valve which is provided at the intake unit of the ventilation exhaust line and is configured to suppress the circulation of the air into the inside of the fuel cell chamber from the outside of the fuel cell chamber in a normal state and to allow the circulation of the air into the inside of the fuel cell chamber from the outside of the fuel cell chamber while being opened when the inside of the fuel cell chamber becomes a negative pressure state; and a chimney unit formed as a double pipe structure including an inner pipe and an outer pipe forming an intake flow channel therebetween, wherein the combustion device is also provided in the indoor place along with the fuel cell system so as to be respectively connected to the intake unit and the exhaust unit of the ventilation exhaust line, the combustion device including a chimney for communication with the chimney unit of the fuel cell system, the intake unit communicates with the intake flow channel and the exhaust unit communicates with the inner pipe, and the first fan is adapted to be driven based on a driving signal for the combustion device even when the fuel cell system is in a standby state. a fuel cell which generates power by fuel and an oxidant gas; a combustion unit which generates a combustion exhaust 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 (11c1) which supplies the oxidant gas to the fuel cell; a housing which accommodates the fuel cell and the combustion unit; an air-tight fuel cell chamber which is provided in an indoor place so as to accommodate at least the housing and the oxidant gas supply device (11c1); a ventilation exhaust line which includes an intake unit introducing air from an outdoor place so as to suction the air into the fuel cell chamber and an exhaust unit discharging the air inside the fuel cell chamber so as to discharge the air into the outdoor place; a combustion exhaust line which includes a cathode air supply pipe causing the inside of the fuel cell chamber and the fuel cell to communicate with each other so that air including the oxidant gas inside the fuel cell chamber is supplied to the fuel cell and an exhaust pipe causing the combustion unit and the outside of the fuel cell chamber to communicate with each other so that a combustion exhaust gas generated in the combustion unit is discharged to the outdoor place; a first fan which is provided at the exhaust unit of the ventilation exhaust line so as to discharge air from the fuel cell chamber; a check valve which is provided at the intake unit of the ventilation exhaust line and is configured to suppress the circulation of the air into the inside of the fuel cell chamber from the outside of the fuel cell chamber in a normal state and to allow the circulation of the air into the inside of the fuel cell chamber from the outside of the fuel cell chamber while being opened when the inside of the fuel cell chamber becomes a negative pressure state; and a chimney unit formed as a double pipe structure including an inner pipe and an outer pipe forming an intake flow channel therebetween, wherein the combustion device is also provided in the indoor place along with the fuel cell system so as to be respectively connected to the intake unit and the exhaust unit of the ventilation exhaust line, the combustion device including a chimney for communication with the chimney unit of the fuel cell system, the intake unit communicates with the intake flow channel and the exhaust unit communicates with the inner pipe, and the first fan is adapted to be driven based on a driving signal for the combustion device even when the fuel cell system is in a standby state.