The relation between the composition of exhaust gas and dioxin concentration was investigated in a pilot scale pyrolysis and smelting system. Increasing of dioxin concentration in post-treatment facilities by De-Novo synthesis was analyzed. Resynthesizing of dioxin was affected by various conditions of combustion and temperature, but direct correlation with concentrations of carbon monoxide and dust was not identified. Additional parameter which affected in resynthesizing of dioxin was thought to be excess oxygen concentration, so operation should be performed in the condition of lower excess oxygen as possible.