PURPOSE: To gasify an inflammable substances, e.g. coal, and to melt or decompose flame-retardant wastes thermally efficiently, by mixing and heating the substances thoroughly from the bottom with a specific molten metal furnace.

CONSTITUTION: Molten steel or steel pieces are fed from the inlets 5 into the molten metal furnace 2 with the plural burners 3 at the bottom, and heated to form the molten metal 4, e.g. molten iron, at 1500W1600°C. A substance to be thermally decomposed, e.g. coal, is fed from the inlets 6 and 7, and air blown from the inlets 8 and 9 to oxidize carbon. The combustible gas formed in the furnace 2 is recovered from the outlet 11 via the flue at the top of the body 1 to give the final fuel gas through the dust removal and purification.