PURPOSE: To obtain reformed methane-rich gas at a low cost, by decarbonating blast furnace gas (BFG) to increase the calorific value, adding carbon dioxide gas obtained thereby to coke oven gas (COG), and carrying out the steam reforming of the COG mixture.

CONSTITUTION: BFG is supplied through the line 1 to the lower part of the carbon dioxide gas absorption column 3, and made to contact countercurrently with the absorbent liquid sent through the line 5. The absorbent liquid absorbed with the carbon dioxide gas is sent through the line 7 to the dissipation column 9, in which the absorbent is separated into carbon dioxide gas and the absorbent liquid. The carbon dioxide gas and the absorbent liquid are discharged through the lines 11 and 5, respecitively. Separately, COG is supplied through the line 13, mixed with carbon dioxide gas supplied through the line 11, and treated in the reforming apparatus 17 in the presence of steam supplied through the line 15. The reformed methane-rich gas converted from COG is taken out of the system through the line 19. The decarbonated BFG is sent to the mixing apparatus 23 through the line 21, mixed if necessary with untreated BFG sent from the line 25, COG sent from the line 27 and/or the reformed gas sent from the line 19, and used as the fuel gas for the coke oven.