NOVELTY - Fuel cell system has hydrogen production device (20) having reforming section (1), shift reaction section (2) and carbon monoxide removing section (3), fuel cell (4), air supply portions (6,9) and impurity removal units (12,13). Cell (4) generates electricity with hydrogen rich gas and oxidizer gas. Portions (6,9) supply air to portion(s) (2) between cell and section (3), or upstream of section (1) based on flow direction of fuel gas.    USE - Used as domestic co-generation fuel cell system.    ADVANTAGE - The fuel cell system maintains stable driving for a long period by removing impurity contained in air, and has improved efficiency.    DETAILED DESCRIPTION - The fuel cell system has a hydrogen production device (20) including a reforming section (1), a shift reaction section (2) and a carbon monoxide removing section (3), a fuel cell (4), air supply portions (6,9) and impurity removal units (12,13). The reforming section produces hydrogen-rich gas containing carbon monoxide from water and hydrocarbon fuel. The shift reaction section produces hydrogen and carbon dioxide from water and carbon monoxide contained in the hydrogen rich gas. The carbon monoxide removing section removes carbon monoxide, which is not removed in the shift reaction section. The fuel cell generates electricity with the hydrogen rich gas fed from the hydrogen production device and an oxidizer gas. The air supply portions supply air to at least one portion (2) between the fuel cell and carbon monoxide removing section, or to upstream of the reforming section based on the direction of flow of fuel gas. The impurity removing section removes any impurity contained in the air.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the fuel cell system. (Drawing includes non-English language text).    reforming section (1)    shift reaction section (2)    carbon monoxide removing section (3)    fuel cell (4)    air supply portions (6,9)    impurity removal units (12,13)    hydrogen production unit (20)