To clarify leakage water of a reclaimed land, and the like, by using a used pollution preventive membrane.

When a reclaimed land is reclaimed, a reticular support frame is constructed in an upper layer, a used polution preventive membrane is constructed on its upper surface, a water filtrating material is further constructed on an upper face of the pollution preventive membrane, and further the pollution preventive membrane is constructed on its upper face to form one unit layer. The method for treating leakage water in the reclaimed land, and the like, is performed by providing single or two or more unit layers.