NOVELTY - A fuel modifier has a modification reaction device comprising a base reactor (1), load response reactors (2-7) and a distributor for supplying raw material containing hydrocarbon, water and air to reactors (1-7). The raw material is continuously supplied to the base reactor and is supplied depending on hydrogen production quantity to the load response reactors.    USE - For manufacturing reformed gas which contains hydrogen from hydrocarbon, air and water, for fuel cell systems used for electric vehicles and co-generation systems.    ADVANTAGE - The catalyst temperature of the load response reactor is always maintained at operating temperature and load response of fuel modifier is increased.    DETAILED DESCRIPTION - A fuel modifier has a modification reaction device, where a raw material, comprising two or three components containing hydrocarbon, water vapor and air, are reacted to form reformed gas comprising hydrogen. A combustor (8) burns combustible gas and supplies heat to the modification reaction device. The modification reaction device comprises a base reactor (1) and load response reactors (2-7) and a distributor for supplying raw material to the reactors (1-7). The raw material is continuously supplied to the base reactor and is supplied depending on hydrogen production property to the load response reactors.    INDEPENDENT CLAIMS are also included for the following:    (i) a fuel cell system including an electrode arranged on both sides of a fuel modifier, a device for carrying out selective oxidation of carbon monoxide in the reformed gas manufactured by the fuel modifier and an electrolyte film, where the fuel cell system has a fuel cell which oxidizes electrochemically hydrogen in the reformed gas and generates electric power and water, and where the reformed gas with low hydrogen concentration ejected from the fuel cell is supplied back to the combustor of the fuel modifier;    (ii) an electric vehicle which has the fuel cell system; and    (iii) a co-generation system comprising a fuel cell system and water cooling-type heat exchanger which cools the reformed gas, where the water ejected from the heat exchanger is used for hot water supply.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the fuel modifier.    Base reactor (1)    Load response reactors (2-7)    Combustor (8)    Switching valves (10,12,14)