TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: Carbon dioxide and carbon monoxide remaining in mixed gas without modification, and/or hydrogen remaining in mixed gas without modification are separated and recovered. The mixed gas is blast furnace gas (BB). Hydrogen is produced using reaction heat produced during conversion of carbon dioxide and/or carbon monoxide. The reaction heat is used as heat source of dehydrogenation of hydride of monocyclic aromatic compound and/or polycyclic aromatic compound under reaction pressure higher than reaction pressure of methanation reaction, when manufacturing hydrogen. The dehydrogenation is performed in a reactor. The produced hydrogen is added to carbon dioxide and/or carbon monoxide without raising pressure. At least one portion of hydrogen is obtained by decomposing ammonia using the reaction heat in the presence of a hydrogen separation film. The reaction heat is used as heat source for pre-heating of hydrocarbon when manufacturing hydrogen by steam reforming hydrocarbon (GG). Water vapor is produced with the reaction heat, and reforming reaction is performed using water vapor. A vapor is generated with the reaction heat, electric power is produced with the vapor, and hydrogen is manufactured by electrolyzing water with electric power or separating hydrogen from hydrogen-containing gas by physical swing adsorption method using electric power. Carbon dioxide is separated and recovered from mixed gas. Hydrogen reducing agent is added to carbon dioxide, and carbon dioxide is converted into carbon monoxide. Water, or water and nitrogen are separated and removed from the resulting gas, and gas is blown into blast furnace. The hydrogen group reducing agent is hydrogen and/or ammonia. The hydrocarbon group reducing agent is gas containing methane or liquefied petroleum gas as main component. The hydrocarbon group reducing agent is gas and/or liquid containing methanol and/or dimethyl ether as main component. The exhaust heat of iron mill is used as one or whole portion of heat source when separating carbon dioxide, or carbon monoxide and carbon dioxide.