A fuel cell system is provided which has a humidifying means (2) for directly humidifying a polymer electrolyte membrane (12) that serves as an ionic conductor. As the humidifying means (2), means is employed which has a water holding unit (21) comprised of a water-absorbing material and located in contact with the polymer electrolyte membrane to directly humidify the polymer electrolyte membrane (12) by utilizing a capillary action and further has a humidification water passage (28) comprised of a hydrophilic material and provided in the polymer electrolyte membrane so as to be connected to the water holding unit (21) to humidify the polymer electrolyte membrane (12) rapidly and uniformly. Such a constitution provides a fuel cell system that can directly humidify an ionic conductor.