In order to develop a safe technology for recycling municipal solid waste incineration fly ash (MSWI-FA), the authors have attempted to produce foamed geopolymers with addition of MSWI-FA, and have investigated their various properties, including bulk density, strength, thermal conductivity, and leaching concentration of heavy metals and chlorine, etc. The polymerization reaction products, crystalline compounds and microstructure were also examined through X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses. It was found that it is possible to produce foamed geopolymer with a bulk density of less than 0.5 and a uniform distribution of air voids, by adding metallic aluminum powder as foaming agent and using less than 20% MSWI-FA and suitable alkali activator having low NaOH mole concentration. CaCl(OH)₂ Ca(OH)₂, CaO and KCl, initially included in the MSWI-FA, were not found in the foamed geopolymers. The geopolymers, which used ground granulated blast furnace slag (BFS), coal fly ah and MSWI-FA as precursors, were mainly composed of C-A-S-H gels, incompletely reacted precursors and a small amount of N-A-S-H gels. The foamed geopolymer had very high immobilization capacity of heavy metals (Cd, Cr, Cu, Mn, Pb, Zn), not varying with the pH of leachate. The immobilization efficiency of As changed with the pH of leachate and BFS content. When the BFS content was not less than 60%, the leaching concentrations of all the traced heavy metals including As were low, satisfying the environmental criteria of Japan for recycled construction materials without direct contact with water. The chlorine immobilization capacity of the foamed geopolymers is expected to exceed 70% in long-term age.