To substantially completely incinerate unburned gas, incompletely burned gas with low incinerating cost by providing a heat generating zone at a suitable position in a chimney of a furnace chamber, and providing a heat generating zone formed in a knitted mesh state at an inlet of a flue. A heat generating zone 5 of a knitted mesh state made of nichrome wires, etc., and a spiral heat generating zone 6 or a honeycomblike heat generating zone are provided at suitable positions where unburned gas, incompletely burned gas is stayed or easily passed in a flue inlet 2, a chimney 3 of a furnace chamber 1. When the zone 5 is provided in the flue inlet and the zone 5 and the honeycomblike zone are provided under the chimney, the unburned gas or the incompletely burned gas can be substantially effectively brought into contact with the zone. When the zone 6 made of the nichrome wires, etc., is installed inside a furnace wall 8, unburned gas or incompletely burned gas stayed in the wall can be positively burned. |