A fuel cell system of the present invention includes: a hydrogen generator (1) having a reformer (1a) configured to generate a hydrogen-containing gas by a reforming reaction using a raw material; a fuel cell (2) configured to generate electric power using the hydrogen-containing gas supplied from the hydrogen generator (1); a combustor (3) configured to combust an anode off gas discharged from the fuel cell (2) to heat the reformer (1a); a CO detector (7) configured to detect a carbon monoxide concentration of a flue gas discharged from the combustor (3); an electric heater (8) configured to heat the CO detector (7); and a controller (19), and the controller (19) is configured to increase an amount of energization to the electric heater (8) in accordance with an increase in an amount of electric power generated by the fuel cell (2).