A fuel cell system is proposed which is provided with a CO oxidizer having a compact agitator executing uniform mixing. The fuel cell system is provided with a reformer (2) performing reformate reactions to produce a hydrogen-rich reformate gas containing CO, a CO oxidizer (1) reducing the CO concentration in the reformate gas produced in the reformer (2), and a fuel cell performing power generation using reformate gas with a reduced CO concentration from the CO oxidizer. The CO oxidizer is provided with a CO removal catalyst (6) promoting oxidizing of CO. An oxidizing agent inlet (5) and an agitator (4) are provided upstream of the CO removal catalyst (6). The oxidizing agent inlet (5) supplies oxidizing agent to the reformate gas and the agitator (4) has a plurality of curved passages (8) having at least two curves.