NOVELTY - A fuel cell system comprises fuel gas supply flow path which supplies hydrocarbon-type fuel gas, fuel gas flow control unit (20) to control supply amount of fuel gas supplied through fuel gas supply flow path, water supply flow path which supplies water for reformation, water amount control unit (26) which controls reformed water supply amount, modifier for carrying out steam reformation of fuel gas from fuel gas supply flow path using water reformed from water supply flow path, and control unit (62) for controlling fuel gas flow control unit and water amount control unit.    USE - A fuel cell system for electric power production (claimed).    ADVANTAGE - Amount of supplied water can be easily controlled, and deterioration of fuel cell can be suppressed.    DETAILED DESCRIPTION - A fuel cell system comprises fuel gas supply flow path which supplies hydrocarbon-type fuel gas, fuel gas flow control unit (20) to control supply amount of fuel gas supplied through fuel gas supply flow path, water supply flow path which supplies water for reformation, water amount control unit (26) which controls reformed water supply amount, modifier for carrying out steam reformation of fuel gas from fuel gas supply flow path using water reformed from water supply flow path, control unit (62) for controlling fuel gas flow control unit and water amount control unit, mass flow rate detection unit for detecting flow volume of fuel gas supplied to fuel gas supply flow path, and heat amount detection unit (19) for detecting amount of heat of supplied fuel gas. Reformed fuel gas reformed by modifier is set as anode gas. Control unit controls fuel gas flow control unit and water amount control unit based on composition of fuel gas to increase amount of reformed water supplied to modifier in transient period before transferring to operation after composition fluctuation. Fuel cell produces electric power by making air into cathode gas.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the control system of the fuel cell. (Drawing includes non-English language text).    Flow volume detection unit (18)    Heat amount detection unit (19)    Fuel gas flow control unit (20)    Water amount control unit (26)    Control unit (62)