To shorten time required for melting, by allowing a material insert pipe to be protruded while penetrating a ceiling of the other end part of a furnace body, and providing a door comprising an exhausting port at the upper end of the material insert pipe for a part of exhaust gas in the furnace to be exhausted, for pre-heating an additional insert material in the material insert pipe for improved thermal efficiency.

Supply of combustion air and sucking of in-furnace exhaust are alternately performed through a heat storage chamber 3 of a regenerative alternate combustion burner 2 provided at one end part of a furnace body 1, and the combustion air is pre-heated with the heat of the in-furnace exhaust. Here, a material insert pipe 5 is so provided as to penetrate a ceiling 4 at the other end part of the furnace body 1, while a door 7 comprising an exhaust port 6 is provided at the upper end of the material insert pipe 5. A part of the in-furnace exhaust is made to exhaust from the exhaust port 6 so that an additional insert material B in the material insert pipe 5 is pre-heated during melting operation for an initial (previous) insert material A. Thus, the heat which escapes from the exhaust pipe for adjusting furnace pressure through the heat storage chamber 3 so far is effectively utilized so that thermal efficiency is improved for shorter time required for melting.