Abstract: Incineration flyash is treated by reducing the organic chlorine compounds contained in the flyash. A cylindrical body equipped with a rotating shaft having a screw conveyor and agitating blades for conveying and agitating the flyash is filled to at least a 75 vol % charge so that contact of the flyash with oxygen is made as small as possible. The flyash is subjected to agitation and heat-treatment at 300.degree. C. or higher. The cylindrical body has a feeding port for the waste incineration flyash and a discharging port for the discharge of the treated flyash. The screw conveyor is at the introducing part and has a pitch that is successively reduced toward the agitating part for charging the flyash in the agitating part at a greater vol. % than that which occurs in the introducing part.