The insoluble treatment of radioactive cesium in municipal solid waste incineration (MSWI) fly ash by soil-mixed geopolymer solidification using Akadama soil, which contained abundant amounts of SiO2 and Al2O3, as an activated filler was investigated. The elution rate of radioactive cesium from the geopolymer solidified substance mixed MSWI fly ash and Akadama soil was suppressed to 0.55%, compared to 69.6% for raw MSWI fly ash, using the Japanese No. 13 leaching test. Almost all radioactive cesium in MSWI fly ash was insolubilized by mixing it with Akadama soil, which showed ability as an active filler and adsorption potential for radioactive cesium. A tank leaching test was applied to confirm the long-term stability of the geopolymer solidified substance. The geopolymer solidified substance showed stability in contact with pure water and on ultraviolet radiation. The geopolymer solidifying method using Akadama soil is effective as an insolubilization treatment of radioactive cesium in MSWI fly ash. This method could be applied to other waste materials.