A fuel cell system comprising: a solid oxide fuel cell which includes a cathode and an anode, and which generates power by using, as fuel, air supplied to the cathode, and a hydrogen-containing gas supplied to the anode; a combustor configured to generate a combustion exhaust gas by combusting an anode-off gas discharged from the anode of the fuel cell and a cathode-off gas discharged from the cathode of the fuel cell; a reformer configured to steam-reform a material, and thereby generate the hydrogen-containing gas to be supplied to the anode of the fuel cell; a first temperature detector configured to detect a temperature of at least one of the combustion exhaust gas and the combustor; and a controller; characterized in that the controller is configured to perform, if the temperature detected by the first temperature detector is lower than a preset first threshold while the combustor is forming flame, at least one of an operation of increasing a ratio of the air consumed by the cathode of the fuel cell to the air supplied to the cathode of the fuel cell, and an operation of decreasing an amount of water supplied to the reformer, the fuel cell system further comprises a second temperature detector configured to detect a temperature of the fuel cell, wherein if the temperature detected by the first temperature detector is lower than the first threshold, the controller is configured to perform the operation of increasing the ratio of the air consumed by the cathode of the fuel cell to the air supplied to the cathode of the fuel cell such that the temperature detected by the second temperature detector does not exceed a preset fourth threshold. a solid oxide fuel cell which includes a cathode and an anode, and which generates power by using, as fuel, air supplied to the cathode, and a hydrogen-containing gas supplied to the anode; a combustor configured to generate a combustion exhaust gas by combusting an anode-off gas discharged from the anode of the fuel cell and a cathode-off gas discharged from the cathode of the fuel cell; a reformer configured to steam-reform a material, and thereby generate the hydrogen-containing gas to be supplied to the anode of the fuel cell; a first temperature detector configured to detect a temperature of at least one of the combustion exhaust gas and the combustor; and a controller; characterized in that the controller is configured to perform, if the temperature detected by the first temperature detector is lower than a preset first threshold while the combustor is forming flame, at least one of an operation of increasing a ratio of the air consumed by the cathode of the fuel cell to the air supplied to the cathode of the fuel cell, and an operation of decreasing an amount of water supplied to the reformer, an operation of increasing a ratio of the air consumed by the cathode of the fuel cell to the air supplied to the cathode of the fuel cell, and an operation of decreasing an amount of water supplied to the reformer, the fuel cell system further comprises a second temperature detector configured to detect a temperature of the fuel cell, wherein if the temperature detected by the first temperature detector is lower than the first threshold, the controller is configured to perform the operation of increasing the ratio of the air consumed by the cathode of the fuel cell to the air supplied to the cathode of the fuel cell such that the temperature detected by the second temperature detector does not exceed a preset fourth threshold.