PURPOSE: To perform fusion solidification treatment efficiently by performing treatment of a material which causes a chemical reaction during solidification using a microwave melting furnace constituted of ≥2 succeeding melting chambers.

CONSTITUTION: A chemical reaction generating decomposed gas is caused in the initial stage of heating for melting in a stage for transforming a material to be treated such as sodium nitrate or sodium sulfate to a vitreous or ceramic solidified product. For the hot-melting treatment of such material, a microwave melting furnace constituted of ≥2 succeeding melting chambers A, B formed by connecting a microwave guide 5 to each melting chamber A, B is used. Reaction is caused by feeding a material to be treated 4 to one melting chamber A, and the reaction product is discharged into another melting chamber B, and vitreous or ceramic melt is discharged from a discharging port 8 of the melting chamber B. By this method, fusion solidification treatment is performed with high efficiency.