FUEL CELL SYSTEM

A fuel cell system, includes: a fuel cell (34) generating electricity using a fuel and an oxidant gas; a reforming unit (33) generating the fuel from a reforming raw material and reforming water, and supplying the fuel to the fuel cell; a combustion unit (36) to which a combustible gas is introduced from the fuel cell to combust the combustible gas with the oxidant gas, and from which a combustion gas is output; a raw material supply device (11a6) supplying the reforming raw material; an oxidant gas supply device (11c1) supplying the oxidant gas; a flow rate detecting sensor (11 a3) detecting a flow rate of the reforming raw material; a first temperature sensor (33a) detecting a temperature of the reforming unit; a second temperature sensor (33b) detecting a temperature of the combustion unit; and a control device (15) determining that a gas component of the reforming raw material varies.