NOVELTY - A fuel reformer has modifier (10), fuel supply unit, modification water supply unit (30), modification water distribution unit (35-37), heat recovery unit, bypass supply unit (31,32) and control unit. Control unit controls fluctuating ratio of total amount of modification water and fuel supplied to modifier, according to amount of required hydrogen. Bypass supply unit operates, when fluctuating ratio is less than useable lower limit.    USE - For fuel cell system (claimed).    ADVANTAGE - The fuel reformer enables the supply of required amount of cooling water to each cooling portion, without using flow-regulating valve. The over cooling of water is prevented.    DETAILED DESCRIPTION - A fuel reformer has a modifier, a fuel supply unit for supplying modification fuel to modifier, a modification water supply unit for supplying modification water to modifier, a modification water distribution unit for distributing supplied modification water as coolant to cooling portions (14,16,18) of modifier at predetermined ratio, a heat recovery unit, a bypass supply unit and a control unit. The modifier produces hydrogen rich gas from modification raw material containing modification water and modification fuel, and is equipped with several cooling portions. The heat recovery unit recovers the heat from modification water supplied to cooling portion. The bypass supply unit directly supplies at least one portion of modification water, according to signal. The control unit controls the supply amount of modification water and modification fuel, according to the amount of required hydrogen. The control unit controls the fluctuating ratio of the total amount of modification water and modification fuel, according to the amount of required hydrogen. The bypass supply unit operates, when the fluctuating ratio is less than the useable lower limit.    An INDEPENDENT CLAIM is included for fuel cell system, which contains fuel reformer as reformed gas supply source.    DESCRIPTION OF DRAWING(S) - The figure shows an explanatory drawing of fuel reformer. (Drawing includes non-English language text).    modifier (10)    cooling portions (14,16,18)    modification water supply unit (30)    bypass supply unit (31-32)    modification water distribution unit (35-37)