A fuel cell system is provided to reduce the operation cost of a fuel cell system and to allow a stack unit to be purged without the damage of a catalyst by recycling a ventilation gas. A fuel cell system comprises a stack unit(130) which generates electricity by the electrochemical reaction of hydrogen and air; an air supply unit(120) which supplies air to the stack unit; a reformation unit(200) which supplies hydrogen to the stack unit; and a heat supply unit(300) which supplies the ventilation gas discharged after the generation of heat to the stack unit, thereby purging the stack unit.