A fuel cell system comprising: a plurality of cell units, including a first cell unit and a second cell unit located below the first cell unit along the vertical direction; and at least one connecting section including a first connecting section to connect the first cell unit and the second cell unit, the plurality of cell units each including: a treatment bath having a flow path through which a liquid to be treated is allowed to flow; a liquid intake port (11A, 11B) through which the liquid to be treated is supplied to the flow path and a liquid discharge port (13A, 13B) through which the liquid to be treated is discharged from the flow path; and at least one electrode cell, the at least one electrode cell including a negative electrode, a positive electrode at least a portion of which is a porous body, and an ion-permeable membrane disposed between the positive electrode and the negative electrode, the ion-permeable membrane being electrically non-conductive, the at least one electrode cell being disposed so that the negative electrode is in contact with the liquid to be treated flowing through the flow path, and that the positive electrode is exposed to a gas phase, and the first connecting section including: a connection path through which the liquid to be treated having been discharged from the liquid discharge port (13A, 13B) of the first cell unit is allowed to flow to the liquid intake port (11A, 11B) of the second cell unit; and a pneumatic adjustment section to suppress pneumatic fluctuations within the connection path that are associated with movements of the liquid to be treated. a plurality of cell units, including a first cell unit and a second cell unit located below the first cell unit along the vertical direction; and at least one connecting section including a first connecting section to connect the first cell unit and the second cell unit, the plurality of cell units each including: a treatment bath having a flow path through which a liquid to be treated is allowed to flow; a liquid intake port (11A, 11B) through which the liquid to be treated is supplied to the flow path and a liquid discharge port (13A, 13B) through which the liquid to be treated is discharged from the flow path; and at least one electrode cell, the at least one electrode cell including a negative electrode, a positive electrode at least a portion of which is a porous body, and an ion-permeable membrane disposed between the positive electrode and the negative electrode, the ion-permeable membrane being electrically non-conductive, the at least one electrode cell being disposed so that the negative electrode is in contact with the liquid to be treated flowing through the flow path, and that the positive electrode is exposed to a gas phase, and a treatment bath having a flow path through which a liquid to be treated is allowed to flow; a liquid intake port (11A, 11B) through which the liquid to be treated is supplied to the flow path and a liquid discharge port (13A, 13B) through which the liquid to be treated is discharged from the flow path; and at least one electrode cell, the at least one electrode cell including a negative electrode, a positive electrode at least a portion of which is a porous body, and an ion-permeable membrane disposed between the positive electrode and the negative electrode, the ion-permeable membrane being electrically non-conductive, the first connecting section including: a connection path through which the liquid to be treated having been discharged from the liquid discharge port (13A, 13B) of the first cell unit is allowed to flow to the liquid intake port (11A, 11B) of the second cell unit; and a pneumatic adjustment section to suppress pneumatic fluctuations within the connection path that are associated with movements of the liquid to be treated. a connection path through which the liquid to be treated having been discharged from the liquid discharge port (13A, 13B) of the first cell unit is allowed to flow to the liquid intake port (11A, 11B) of the second cell unit; and a pneumatic adjustment section to suppress pneumatic fluctuations within the connection path that are associated with movements of the liquid to be treated.