To maintain a smooth flow of an exhaust gas discharged from a preheating device and the functions of a deodorizing tower, dust collector, etc., by catching small pieces of metallic scraps contained in the exhaust gas and, at the same time, burning the carbon monoxide contained in the exhaust gas in a buffer tower.

A preheating device 21 is installed to the top of a melting furnace 11 and a buffer tower 51 is connected to the exhaust gas duct 41 of the preheating device 21. In the tower 51, the exhaust gas introduced from an exhaust gas inlet 53 formed through the lower section of the side wall of a tower 52 of a closed system is discharged from an exhaust gas outlet 53 after rising in the tower 52. While the exhaust gas rises in the tower 52, small pieces of metallic scraps contained in the exhaust gas are caught and accumulated on the bottom of the tower 52 by utilizing their own weights. In addition, after the carbon monoxide contained in the exhaust gas is burned with burners 54 and 55 and the odorant components and oil components are thermally decomposed, the exhaust gas is cooled with a cooler 71 on the downstream side and the dust, etc., contained in the exhaust gas are removed by means of a dust collector 81. Therefore, a smooth flow of the exhaust gas can be maintained.