NOVELTY - A fuel cell system comprises a branch fuel gas flow passage (44) connecting a fuel gas supply passage to a combustor (42a), and comprising a fuel gas supply quantity control unit for controlling a supply quantity of the fuel gas to be supplied into the combustor; and a pressure control unit (47) provided upstream of the fuel gas supply quantity control unit, for controlling a pressure in the fuel gas according to a pressure in a branch oxidant gas flow passage (48, 51).    USE - Used e.g. as proton exchange membrane fuel cell system for a fuel cell-driven automobile (vehicle), marine vessel, or an aircraft.    ADVANTAGE - The system is capable of appropriately controlling combustion even when the system starts up at low temperature, and of efficiently warming up a fuel cell. Because no oil-gas is used, each quantity of the fuel gas and the oxidant gas supplied into the combustor can be accurately measured, allowing an easy control of the combustion at a target temperature. The combustor can be operated even when the fuel cell is not in operation of generating electricity, and can effectively warm up the fuel cell. When a valve which can completely shut off a flow passage is used as the cathode gas flow rate control valve, it is possible to prevent air from leaking into the branch cathode gas supply pipe to prevent a wasteful energy consumption of the air compressor.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell for generating electricity from a fuel gas and an oxidant gas supplied; a fuel off-gas supply passage connecting a fuel gas supply with the fuel cell; an oxidant gas supply passage connecting an oxidant gas supply with the fuel cell; a combustor for combusting fuel gas and oxidant gas; a branch fuel gas flow passage connecting the fuel gas supply passage to the combustor; a first branch oxidant gas flow passage connecting the oxidant gas supply passage with the combustor; a fuel off-gas flow passage for exhausting fuel gas from the fuel cell; an oxidant off-gas flow passage for exhausting oxidant gas from the fuel cell; a first heat exchanger associated with the combustor for transferring heat generated by combustion in the combustor to a heat transfer medium; and a heat transfer medium circulation passage connecting the first heat exchanger to the fuel cell for circulating the heat transfer medium between the first heat exchanger and the fuel cell.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram illustrating an entire fuel cell system.    Combustor (42a)    Branch fuel gas flow passage (44)    Pressure control unit (47)    Branch oxidant gas flow passage (48, 51)