One end of an air passage (32) facing a cathode (33B) of a fuel cell (36) forming a fuel cell stack (6) is connected to a first manifold (1A), and the other end is connected to a second manifold (1B). A water-absorbing material (30) is disposed at a position in contact with an air flow in the first manifold (1A) and second manifold (1B). The air is made to circulate according to a humidity state by changing over between a first air flow direction wherein air flows from the first manifold (1A) to the second manifold (1B) via the air passage (32), and a second air flow direction wherein air flows from the second manifold (1B) to the first manifold (1A) via the air passage (32), so a membrane electrolyte is efficiently humidified.