NOVELTY - A fuel cell system comprises modification part (1) which contains modification catalyst and generates reformed gas containing hydrogen by steam reforming reaction of vaporized water from water supply apparatus (5) and source gas from source gas supply apparatus (4) in the presence of modification catalyst, hydrogen generating apparatus provided with selective oxidizing part (3) which contains selective oxidation catalyst, and stack (10) which produces electric power using oxidizing agent gas and hydrogen-containing gas supplied from hydrogen generating apparatus.    USE - A fuel cell system e.g. domestic fuel cell system.    ADVANTAGE - Amount of carbon monoxide in gas generated from hydrogen producing apparatus can be stably reduced, fuel cell system can be utilized over a long period of time, and stable electric power generation can be performed.    DETAILED DESCRIPTION - A fuel cell system comprises modification part (1) which contains modification catalyst and generates reformed gas containing hydrogen by steam reforming reaction of vaporized water from water supply apparatus (5) and source gas from source gas supply apparatus (4) in the presence of modification catalyst, hydrogen generating apparatus provided with selective oxidizing part (3) which contains selective oxidation catalyst and reduces carbon monoxide in gas mixture containing air from 1st air supply apparatus and gas flowing from modification part by selective oxidation reaction, and stack (10) which produces electric power using oxidizing agent gas and hydrogen-containing gas supplied from hydrogen generating apparatus. Hydrogen producing apparatus temporarily reduces flow volume of air from air supply apparatus from predetermined value, when source gas is supplied from source gas supply apparatus, water is supplied from water supply apparatus, and air is supplied from 1st air supply apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the hydrogen producing apparatus. (Drawing includes non-English language text).    Modification part (1)    Selective oxidizing part (3)    Source gas supply apparatus (4)    Water supply apparatus (5)    Stack (10)