Abstract: A fuel cell system includes a fuel cell stack, a heat exchanger for heating an oxygen-containing gas using a heat medium before the oxygen-containing gas is supplied to the fuel cell stack, a reformer for reforming a raw fuel chiefly containing hydrocarbon to produce a fuel gas to be supplied to the fuel cell stack, a combustor for burning a raw fuel and an exhaust gas discharged from the fuel cell stack after consumption in power generation reaction to produce a combustion gas, and a heat retention chamber provided to cover opposite ends of the fuel cell stack in the stacking direction. Before an exhaust gas is supplied to the combustor, the exhaust gas flows into the heat retention chamber as a heat source for maintaining the temperature of the fuel cell stack.