A cell assembly is formed by stacking a first cell and a second cell. In
the cell assembly, oxygen-containing gas flow passages are connected in
series by an intermediate oxygen-containing gas flow passage, and fuel
gas flow passages are connected in series by an intermediate fuel gas
flow passage. An additional oxygen-containing gas is supplied to an
oxygen-containing gas passage which includes the intermediate
oxygen-containing gas flow passage. An additional fuel gas is supplied to
a fuel gas passage which includes the intermediate fuel gas flow passage.