A fuel cell system (100) comprising: a fuel cell (1) for generating electric power using a fuel gas; a fuel gas generator section (2) for generating the fuel gas using a raw material gas; a combustor section (2a) for generating heat energy to be used for generating the fuel gas; an air feeder section (2b) for feeding aft when the heat energy is generated; and a controller section (101); wherein the fuel cell is filled with the raw material gas before the fuel gas is fed, and wherein the controller section (101) performs a control operation such that the feed rate of air supplied from the air feeder section (2b) to the combustor section (2a) increases when the fuel gas generated in the fuel gas generator section (2) is supplied to the fuel cell (1).