A fuel cell system (100) comprises: a plurality of cell units including a first cell unit (10A) and a second cell unit (10B) positioned below the first cell unit (10A) in the vertical direction; and at least one connection portion including a first connection portion (20A) for connecting the first cell unit (10A) and the second cell unit (10B). In the fuel cell system (100), the cell units each have at least one electrode cell (2) equipped with: a processing bath (3) having a flow path (8) for circulating a liquid to be processed; a liquid supply inlet (11A, 11B) for supplying the liquid to be processed to the flow path (8); and a liquid discharging outlet (13A, 13B) for discharging the liquid to be processed from the flow path (8). The first connection portion (20A) has: a connection path (9) for circulating the liquid to be processed discharged from the liquid discharging outlet (13A, 13B) of the first cell unit (10A) to the liquid supply inlet (11A, 11B) of the second cell unit (10B); and an atmospheric pressure adjustment portion for suppressing atmospheric pressure variations caused by the movement of the liquid to be processed in the connection path (9).