NOVELTY - A fuel cell system comprises hydrogen generating apparatus (1) which generates fuel gas containing hydrogen by reforming reaction using source gas, reactive gas supply device which supplies gas required for reforming reaction to hydrogen generating apparatus, combustor (2) which burns gas sent from hydrogen generating apparatus, and controller which performs combustion process of gas or dilution process which flows in into combustor while blocking first path by first valve after stopping supply of reactive gas from reactive gas supply device.    USE - A fuel cell system useful as co-generation system.    ADVANTAGE - The fuel cell system reduces gas volume which flows in into fuel gas flow channel in fuel cell when processing gas which flows in into combustor after reactive gas supply stoppage to hydrogen generating apparatus, flooding generated in fuel gas flow channel in fuel cell at the time of generating mode and fall of electric power generation amount.    DETAILED DESCRIPTION - A fuel cell system comprises hydrogen generating apparatus (1) which generates fuel gas containing hydrogen by reforming reaction using source gas, reactive gas supply device which supplies gas required for reforming reaction to hydrogen generating apparatus, combustor (2) which burns gas sent from hydrogen generating apparatus, fuel cell (4) which is generated using fuel gas sent from hydrogen generating apparatus, first path which supplies gas sent from hydrogen generating apparatus to combustor via fuel cell, second path which reroutes gas supplied to combustor via fuel cell, first valve which connects/blocks first path, second valve which connects/blocks second path, and controller which performs combustion process of gas or dilution process which flows in into combustor while blocking first path by first valve after stopping supply of reactive gas from reactive gas supply device. The generating mode is performed in state which blocked second path by second valve.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the fuel cell system. (Drawing includes non-English language text).    Hydrogen generating apparatus (1)    Combustor (2)    Ignition device (3)    Fuel cell (4)    Fuel gas supply path (5)