To provide a method for vitrification of chlorine-contg. residual substance from flue gas purification wherein chlorine can be enough transferred to vapor phase at a relatively low processing temp.

In order to vitrify chlorine-contg. residual substance from flue gas, the residual substance is melted in a melting furnace 8 and is successively taken out. For conditioning of this melted liq., an alkaline flux is added as a form of oxide, hydroxide or carbonate so as to make the stoichiometric ratio of the sum of alkali to chlorine larger than 0.75. Oxide, hydroxide or carbonate of sodium or potassium or a mixture of these substances are especially pref. It is possible by this method to process a mixture of residual substances with high chlorine content under an economical condition.