TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The reaction heat is supplied side by side to high temperature operation fuel cell or gas turbine which operates using the source gas having lower hydrocarbon as main component, as fuel. The radiant heat of high temperature operation fuel cell main component, or gas turbine main component and/or sensible heat of exhaust gas of high temperature operation fuel cell or gas turbine is utilized.The unused hydrogen in the source gas is selectively separated, and mixed with anode side exhaust gas of high temperature operation fuel cell. A portion or whole of hydrogen in residual gas is mixed with non-converted lower hydrocarbon, after selectively separating aromatic compound from aromatic compound and hydrogen mixture.Carbon precipitated on the catalyst is removed by periodically burning catalyst with mixed gas containing nitrogen monoxide (5-15 %) and oxygen-containing gas (1-50 %, preferably 3-30 %), such as air.Hydrogen or hydrogen-containing gas is mixed with source gas. The source gas is burnt using the reaction heat which is supplied by combustion heat.The synthetic gas modifies the source gas by steam reforming process, partial-oxidation method or carbon dioxide reforming process, and generates hydrocarbon fuels, such as methanol, gasoline and dimethyl ether. The gas having lower hydrocarbon or hydrocarbon fuel is modified using the exhaust heat.Preferred Gas: The exhaust heat of manufacturing process, such as coal gasification gas, coke oven gas, biomass gasification gas, waste material gas and various thermal decomposition heat is utilized for source gas modification and component adjustment. The source gas is obtained by mixing hydrogen produced by gas modification, and component adjustment of coal gasification gas, coke oven gas, biomass gasification gas, waste material gas and various thermal decomposition heat, with gas having lower hydrocarbon as main component. Preferred Composition: The hydrogen or hydrogen-containing gas contained in source gas is 1-30 %, preferably 3-10 %, with respect to lower hydrocarbon and the hydrogen concentration is 5-7 %.Preferred Catalyst: The catalyst comprises a metal chosen from iron, nickel and/or cobalt, a metallic compound chosen from molybdenum and its compound, and metallosilicate.