To improve the durability of a water-repellent surface by exposing the surface to an acid solution and activating the surface and then applying a water-repellent film on the surface.

A water-repellent film is formed by applying a water-repellent composition on a base, and in that case, the base is activated by using an acid solution before applying the water-repellent composition to increase the degree of durability of the water-repellent film. Although the composition of the base is not particularly restricted, bases of uncoated glass, a metal, a plastic, enamel and a ceramic are included in the range of bases to be used. As the water- repellent composition, a composition containing perfluoroalkyl alkylsilane is preferred. As for the acid solution to be used, the selection is made based on the capability of increasing the degree of durability of water repellency of the base without damaging the base, and the solution of hydrochloric acid, sulfuric acid or an organic acid, or phosphoric acid, hydrobronic acid, nitric acid, acetic acid, trifluoroacetic acid, citric acid or the like can be used.