An intermediate plate is interposed between first and second sub-stacks. The intermediate plate has, defined in a surface thereof, an oxygen-containing gas mixing passage interconnecting an oxygen-containing gas outlet in the first sub-stack which is located upstream and an oxygen-containing gas inlet in the second sub-stack which is located downstream. In the first and second sub-stacks, an oxygen-containing gas is supplied from the oxygen-containing gas inlet and discharged to the oxygen-containing gas outlet at all times.