To obtain a slag of a stable glass composition by reducing a supply power to a first melting electrode provided at a position near a slag exhaust port smaller than a supply power to a second melting electrode provided at a remote position, and relatively reducing the supply amount of a refuse incineration residue to the vicinity of the first electrode.

A slag exhaust port 14, used also as a slag hole, is formed at one side of a furnace container 11, and a refuse incinerating residue charging port 15 is provided at the opposite side of a furnace cover 12 to the port 14. A first melting electrode 13C is installed at a position relatively close to the port 14 on a line for perpendicularly crossing a line for connecting the port 14 to the port 15, and a pair of second melting electrodes 13A are provided at relatively remote positions to the port 14. It is controlled such that the supply power to the electrode 13C is reduced smaller than that to the electrode 13A and a supply amount of the residue to the vicinity of the electrode 13C is smaller than that to the vicinity of the electrode 13A.