PURPOSE: To restrict the upward flow velocity of exhaust gas in a furnace and effectively restrain dust from diffusing by dividing the flow of combustion exhaust gas into upper and lower flows.

CONSTITUTION: A part of the combustion exhaust gas from a high-temperature furnace bed 6 is discharged to the outside of the furnace 1 through a molten-material discharge passage 20, while the remainder of the exhaust gas is discharged via the upper part of the furnace 1. By dividing the flow of the exhaust gas into the upper and lower flows, the upward flow velocity of the exhaust gas in the furnace 1 is restricted, the dust is effectively restrained from diffusing and the cramming of the molten material is prevented by the warming effect of the exhaust gas. In addition, stable combustion is securely performed by reducing the heat absorption due to a reducing reaction.