Abstract Fly ash discharged from a municipal refuse incineration plant is regulated, by 1992 legislation, to be treated to prevent leaching of heavy metals before final disposal. We have developed a stable and simple fly ash treatment method “Acid Extraction-Sulfide Stabilization Process (AES Process)”, in which the fly ash is acid-extracted and stabilized by NaHS to change the residual heavy metal into insoluble heavy metal sulfide. The results of many fundamental experiments and pilot tests indicate that it is important to consider the characteristics of fly ash (fine, scattering, moisture-absorbing, etc.) in designing the AES Process. Since 1977, seven municipal incineration plants with AES Process have been constructed. At one of these plants, the fly ash and waste water is treated together by the AES Process and evaporation/crystallization process. The fly ash is discharged as stabilized cake for final landfill disposal; solid salt is recycled to soda production plants and condensed water is recycled in the plant as cooling water. This plant discharges no waste water. Operating results of the AES Process is that the stabilized cake conforms to the standard for landfill disposal in T-Hg, Cd and Pb, and the recovered solid salt contains approximately 99% NaCl. The operation cost per ton of the fly ash is ¥3,840 per ton.