A fuel cell system includes: a solid oxide fuel cell generating power by using, as fuel, air supplied to a cathode and hydrogen-containing gas supplied to an anode; a combustor generating a combustion exhaust gas by combusting anode-off gas and cathode-off gas discharged from the anode and the cathode, respectively; a reformer steam-reforming a material to generate the hydrogen-containing gas supplied to the anode; a first temperature detector detecting temperatures of the combustion exhaust gas and/or the combustor; and a controller performing, if a temperature detected by the first temperature detector is lower than a preset first threshold while the combustor is forming flame, at least one of operations of: increasing a ratio of air consumed to the air supplied in the cathode; decreasing a ratio of hydrogen-containing gas consumed to the hydrogen-containing gas supplied in the anode; and decreasing an amount of water supplied to the reformer.