A method for removing a stain from a surface of a structure which includes the steps of spraying an aggregating agent comprising a cationic linear organic polymer onto the surface of such structure having a stain produced thereon, dissociating the stain particles by use of Coulomb force f generated between the stain particles and the aggregating agent, displacing slightly the stain particles along the surface of the structure to cause peeling thereof from the surface to be cleaned, and simultaneously, aggregating the same for cleaning. Another embodiment includes maintaining water permeability of a road by easily eliminating a stain adhering to the surfaces of granular aggregate particles forming a drainage-type pavement layer of a road without the use of various mechanical tools. Other embodiments include the steps of depositing an aggregating agent for soil improvement comprising a cationic organic polymer onto a drainage-type pavement layer of a road, and eliminating the aggregated stain particles adhering to the surfaces of the granular aggregate particles constituting the drainage-type pavement layer and settling in voids.