A liquid compound or a compound solution is kept in contact with the surface of a solid material, such as a synthetic resin, glass, metal, or ceramic, radiation selected from ultraviolet radiation, visible radiation, and infrared radiation is irradiated on the interface between the surface of the solid material and the liquid compound or compound solution to optically excite the surface of the solid material and the liquid compound or compound solution, thereby effecting substitution with a chemical species in the liquid compound or compound solution, depositing the chemical species, or performing etching with the chemical species. Preferably, a transparent window is kept in tight contact with the surface of a solid material to be treated, a thin layer of a liquid compound or of a compound solution is interposed between the surface of the solid material and the transparent window by using capillarity, and ultraviolet radiation, visible radiation, or infrared radiation is irradiated through the transparent window. With this treatment, the irradiated portion can be imparted with hydrophilicity, adhesion properties printing properties, a corrosion resistance, and conductivity.