A fuel cell system includes: a fuel cell that has a cathode and an anode and generates electricity by reducing a mediator at the cathode; a regenerator that oxidizes, with an oxidant, the mediator reduced by the cathode; a reformer; a combustor that heats the reformer; and a heating medium path that heats the regenerator, wherein through the heating medium path, combustion exhaust discharged from the combustor or a heat medium heated through heat exchange with the combustion exhaust flows.