A single fuel cell stack includes a plurality of divided fuel chambers and a plurality of divided air chambers partitioned or a plurality of fuel cell stacks are connected to each other so that respective fuel chambers and air chambers are connected in series to form a plurality of series fuel chambers and a plurality of an series air chambers. Reaction gas is supplied to selected divided fuel chambers and divided air chambers, or to selected series fuel chambers or series air chambers, selected depending on the stage of load factor.
