It is still a problem concerned that the radioactive Cs is concentrated in MSW incineration residues such as fly ash. Therefore, further developments in incineration technology and management for the MSW containing radioactive materials are needed. In this study, the radioactive Cs and other elements were measured at two types of MSW incineration plants that were a stoker furnace with ash melting furnace, and a gasification and melting furnace. The measurements mainly covered the influence of the change in the composition of the waste incinerated and the ratio of the bottom ash and the fly ash to be melted in the ash melting furnace. The results obtained are;1) Si affects the fate of radioactive Cs in the bottom ash and slag, and Cl affects that in the fly ashes both from incineration and melting process, 2) Radioactive concentration of the slag increases with the decrease of the slag basicity defined as the ratio of CaO/SiO_2, which may suggest that the basicity is a practical index for the radioactive concentration in the produced slag.
