To simply, easily and stably control a temperature when a temperature in a combustion melting furnace becomes high due to change of quality or amount of waste or other cause by lowering the temperature in the furnace by supplying excess combustion air at the time of exceeding a predetermined value at the temperature in the furnace.

Waste (a) heated by heated air (g) is thermally decomposed to become thermal decomposition gas G and decomposed residue (b) containing nonvolatile components, and separated by an exhaust unit 6. The gas G is supplied to a burner 14 of a combustion melting furnace 13. When the temperature of the furnace 13 becomes high exceeding a predetermined value, it is sensed by a temperature sensor in the furnace, and combustion air (e) burnt, for example at an air ratio of 1.3 is supplied excessively exceeding this air amount. To increase the supply amount of the air (e), a volume of a forced draft fan 18 is previously set to high, and the air is blown from blowing ports 19a to 19d by considering a rising ratio of the temperature in the furnace and a temperature distribution in the furnace and considering a balance between the blowing amount and blowing positions.