An object of the present invention is to provide a fuel cell system for preventing carbon deposition in a fuel cell stack to be supplied with reformed gas. A fuel cell system 10A of the present invention includes a partial oxidation reformer 22 for partially oxidizing raw fuel to produce carbon monoxide and hydrogen, a shift reactor 23 for shift reacting the carbon monoxide with steam to produce carbon dioxide and hydrogen, a fuel cell stack 20 for generating electric power by electrochemical reaction between oxidant gas and the hydrogen which is produced in at least one of the partial oxidation reformer 22 and the shift reactor 23, and an exhaust gas recirculation pipe P6 for supplying steam contained in exhaust gas of the fuel cell stack 20 to the shift reactor 23.