The present invention provides a surface-treating agent to form fine roughness on the surface of a material and more specifically a surface treating-agent which forms fine roughness on the surface of a material and is easy to process, thereby being useful for materials for highly water-repellent glass, lenses and fabric, materials with an excellent anti-soiling property, panels having an excellent light scattering property, illumination of optical fiber and the like, materials and coatings to prevent accumulation and adhesion of snow or icicle formation on antennas, wires and steel towers, and roughness formation on the surface of semiconductor substrates; the treated materials; and a method of surface treatment to develop the roughness. The surface-treating agent of the present invention has an average primary particle diameter in the range of 1-50 nm, contains fine particles in the range of 5-60% by mass of the total amount of the surface-treating agent in a slurry of nanoparticles which are treated for water repellency and mechanically dispersed in a solvent containing a volatile solvent, and forms a roughness structure with upward protrusions having a spatial periodicity of 0.1-50 μm on the surface of a material by volatilizing the solvent or dipping repeatedly in water upon treating the surface of the material.