TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Apparatus: The combustor is arranged between the modification reaction devices and is in contact with the base reactor (1) and two load response reactors. The load response reactors are arranged at an equal distance from the base reactor. The base reactor and the combustor have a double pipe structure where the base reactor and combustor are arranged to the inner and outer side, respectively. The base reactor and load response reactor are arranged in one combustor. The distributor has a switching valve (10,12,14) and a rotary valve which are controlled depending on the hydrogen production quantity. The distribution of the load response reactor w.r.t the base reactor is symmetrical. The length of the modification reaction device is higher than that of the combustor. A noble metal catalyst and modification catalyst which contains copper and zinc or nickel are arranged to the raw material inlet and outlet side of the modification reaction device. A heat exchanger is provided on a layer containing a copper-zinc catalyst in the modification reaction device. Hydrocarbon is reacted in the upstream side of the heat exchanger and reformed gas is manufactured. Carbon monoxide and water vapor in the reformed gas is converted into hydrogen and carbon dioxide in the double stream side of the heat exchanger. The heat carrier of the heat exchanger is air. Air in the raw material is heated previously and the temperature of the reformed gas is reduced with the heat exchanger. The hydrocarbon is methanol or natural gas. The fuel cell system has a measuring device to calculate the heat of combustion of the reformed gas, ejected from the fuel cell and a device for measuring the temperature of the fuel modifier. The controller of the fuel cell system determines the amount of raw material supplied to the fuel modifier and distributor, based on electric power load requirement, heat of combustion, temperature of fuel modifier and flow rate.