A cell assembly has reactant gas passages connected in series in first and second unit cells, and oxygen-containing gas inlets, intermediate oxygen-containing gas inlets, oxygen-containing gas outlets, intermediate oxygen-containing gas outlets which are defined in the first and second unit cells. The oxygen-containing gas inlets and the intermediate oxygen-containing gas inlets are disposed upwardly of the oxygen-containing gas outlets and the intermediate oxygen-containing gas outlets. The oxygen-containing gas outlets and the intermediate oxygen-containing gas outlets are disposed at least partly below electric energy generating surfaces of the first and second unit cells.