A landfill experiment was conducted to clarify the leachability and possibility of biodegradation in a landfill site through leachate quality analysis using a large-scale lysimeter. This lysimeter had a cover soil layer consisting of “White-rot fungi” and a waste layer consisting of fly ash including high concentrations of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/PCDFs), bottom ash, and the crushed residue of incombustibles. In this study we also investigated the fundamental properties of white-rot fungi (Ceriporia sp. MZ-340) in landfilled waste layers and the contaminated soil from the detection of cell bodies through genetic analysis and monitoring. From a study of leachate quality analysis of a large-scale landfill experiment carried out over 3 years, the amount of PCDDs/PCDFs moving into the leachate from the landfilled waste layer in the experimental lysimeter was only 1.15 X 10-4%. This means that most of the PCDDs/PCDFs remain in the waste layer. In the case of the elution of the leachate, PCDDs/PCDFs tend to be detected in insoluble materials，, such as suspended solids. Further more, in the third year of observation, almost none of the PCDDs/PCDFs were detected in the leachate. It can therefore be concluded that most of the PCDDs/PCDFs remained in landfill sites. It is therefore important to prevent PCDDs/PCDFs from leaching by focusing on counter measures for the removal of suspended solids in the initial stage of landfilling. Genetic analysis for the detection of white-rot fungi also showed that there is no organism capable of surviving the conditions of the waste landfill layer. As a result of a fundamental degradation experiment, that showed that the growth potential of white-rot fungi is high if suitable conditions have been met, and that the number of fungi reaches its maximum in a short period of time, even when combined with fly ash. It is essential to consider the setting conditions for bioremediation.