To enable longtime maintenance of a surface whereon a waterdrop hardly sticks, by a method wherein the surface of a base is made to have a structure wherein a hydrophilic part wherein photocatalytic oxide particles are exposed and a water-repellent part wherein water-repellent fluoroplastic is exposed are dispersed microscopically in the surface and a contact angle formed by the surface and water is made a specified value.

A large number of fins 2 fitted to a pipe 1 oppositely are made of aluminum flat plates and a surface layer 3 is formed on the surface of each fin. The surface layer 3 has a structure wherein both of a part presenting hydrophilic properties wherein a photocatalytic titanium oxide is so exposed as to be in contact with the air and a part presenting water repellency wherein water-repellent fluoroplastic is so exposed as to be in contact with the air are dispersed microscopically in the surface, and a contact angle formed by the surface and water is made 90° or above. When a light (ultraviolet ray) of a wavelength enabling excitation of the photocatalytic titanium oxide is cast on the surface layer 3 from a light source 4 constructed of an ultraviolet lamp, a fluorescent lamp or the like, the part presenting the hydrophilic properties wherein the photocatalytic titanium oxide is so exposed as to be in contact with the air recovers the hydrophilic properties.