To immobilize lead compounds in fly ash from a melting process of MSW incineration fly ash, fly ash was mixed with waste plastic, sulfur and calcium hydroxide in a bi-axial kneader at 523 K. The waste plastic samples used were: polyethylene (PE), polypropylene (PP) and polystyrene (PS). The physical immobility of treated fly ash with plastics and the applicability of plastics to sulfurating medium were examined by using a dissolution test. As a result, lead concentration in the eluant decreased with an increase in the amount of plastic added. However, Japan's lead emission standard of 0.3 mg/l was not achieved by this treatment. On the other hand, there was no detectable lead in the eluant from fly ash mixed with plastic (2.5 g/10 g-fly ash), sulfur (1.2-1.4 g/10 g-fly ash) and calcium hydroxide (1.0-1.4 g/10 g-fly ash), confirming the complete insolubilization of lead compounds in fly ash.