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, 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 condensing mechanism using the oxygen-containing gas as the coolant. The air cooling condensing mechanism includes a secondary battery for inducing endothermic reaction during charging and inducing exothermic reaction during discharging.