A fuel cell device includes a solid oxide fuel cell; an air-tight housing accommodating the solid oxide fuel cell; a gas concentration detector detecting a gas concentration of a combustible stagnant gas; and a controller performing an emergency stop by instantly stopping the supply of the fuel to the fuel electrode when the gas concentration is higher than an upper limit concentration and performing a normal stop by lowering a temperature of the fuel cell device while supplying the fuel to the fuel electrode when the gas concentration is not higher than the upper limit concentration and the gas concentration is higher than a predetermined concentration.