A fuel cell system of the invention includes outer-conduit water absorbing members that are arranged outside oxidizing gas conduits of respective unit fuel cells, and a changeover mechanism that changes over the status of the outer-conduit water absorbing members between an absorption state in which the outer-conduit water absorbing members absorb moisture in the oxidizing gas conduits and a non-absorption state in which the outer-conduit water absorbing members do not absorb the moisture in the oxidizing gas conduits. Control of this changeover mechanism effectively regulates the moisture level in the oxidizing gas conduits.