FUEL CELL SYSTEM WITH FLAMMABLE GAS SENSOR

A fuel cell system (100) includes a fuel cell (34) which generates electricity by fuel supplied to an anode (34p) and an oxidant gas supplied to a cathode (34q), a combustion portion (36) which burns an anode off gas from the fuel cell (34) and a cathode off-gas from the fuel cell to generate a combustion exhaust gas, a casing (31) formed by a heat-insulating material and accommodating therein the fuel cell (34) and the combustion portion (36), a housing (10a) which accommodates the casing (31) therein and provided with a ventilation discharge port (10d) discharging the air ventilated inside the housing (10a) out of the housing (10a), a discharge passage (50) connected to the casing (31) and discharging the combustion exhaust gas flowing from the casing out of the housing (10a), a flammable gas sensor (58) provided at a sensor arranging portion (60) positioned in the discharge passage (50) at downstream side of a merging point (57) of the ventilated air and the combustion exhaust gas for detecting a concentration of the flammable gas, an instructing portion (15) instructing the flammable gas to flow through the sensor arranging portion (60) with an instruction concentration (C1) and a judging portion (15) which judges whether or not a detection sensitivity of the flammable gas sensor (58) is deteriorated based on an output value (V1) of the flammable gas sensor (58) relative to the instruction concentration (C1).