A fuel cell system includes a fuel cell module for generating electrical energy by electrochemical reactions of a fuel gas and an oxygen-containing gas, and a condenser for condensing water vapor in an exhaust gas discharged from the fuel cell module by heat exchange between the exhaust gas and a coolant to collect the condensed water and supplying the collected condensed water to the fuel cell module. The condenser includes an air cooling condenser using the oxygen-containing gas as the coolant and a water cooling condenser using hot water stored in a hot water tank as the coolant. A thermoelectric conversion mechanism for performing thermoelectric conversion by a temperature difference between the exhaust gas and the oxygen-containing gas is provided between the air cooling condenser and the water cooling condenser.