Radionuclide contamination from the nuclear accident at the Fukushima Daiichi Nuclear Power Plant has been found in sewage sludge ash produced in eastern Japan. When such contaminated waste contains less than 8,000 Bq/kg radiocesium, it is being disposed in controlled landfill sites. In order to assess the possible spread of the radionuclides by their leaching from the landfill sites, it is important to know the leaching behavior of the radionuclides from the sewage sludge ash and factors influencing the leaching behavior. In this study, leaching experiments using stable Cs and Sr were conducted for sewage sludge ash under several conditions to investigate effects of chemical composition of leachate, pH, and solid/liquid ratio on Cs and Sr leaching behaviors. In the pH range from 6 to 12, the leaching ratio of Cs or Sr was less than 5.2 or 0.21%, respectively. Additionally, the leaching ratio of Sr decreased with increasing pH of the leachate. In contrast, the higher the pH in the leachate was, the higher the leaching ratio of Cs was. Finally, possible radionuclide leaching from contaminated sewage sludge ash and then radionuclide concentrations in an actual landfill leachate were assessed. It could be suggested that ⁹⁰Sr leaching from the landfill site had the least effect on the environment, whereas ^134+137Cs leaching needed to be taken into account for spreading radioactive materials from the landfill site to the environment.