To prevent the generation of an organic substance, such as dioxin, in combustion gas by a method wherein a furnace temperature in a burning melting furnace is controlled in relation to a dwelling time of furnace combustion gas.

By effecting combustion treatment controlled so that a formula of (T-1,200)2.0×Q≥20.8×104 is established provided a melt temperature in a combustion melt furnace is T (°K) and a dwelling time of furnace combustion gas is Q(sec), combustion gas wherein the generation of dioxin is reduced is generated. The dwelling time (sec) of combustion gas is indicated a lateral bar and a temperature (°K) of combustion gas is indicated by a longitudinal bar. A dotted line M indicates a curve of an organic material being 0.5ng/Nm3- TEQ or a solid line indicates a curve of 0.1ng/Nm3-TEQ. A range below the doted line M is a region where dioxin is generated and a range above the solid line N and shown by an arrow mark A is a region where the generation of dioxin is low. Thus, a combustion condition is controlled so that the region is above the solid line N.