The present invention provides a gradually soluble coating film to be applied to instruments for waterworks. In a conventional polyvinyl alcohol solution not containing metal salt, there is formed a coating film in which polyvinyl alcohol molecules are piled up as shown in FIG. 2(a). In such coating film, water contacting with a surface thereof penetrates rapidly to the interface between the coating film and a base material, so that when water flows away, the coating film flows out and vanishes completely. On the other hand, in a coating film of the present invention, there are formed polyvinyl alcohol inclusion metal hydroxide clusters derived from the kind of metal in the solution as shown in FIG. 2(b). Although polyvinyl alcohol is dissolved or swollen to allow water to penetrate therein when water contacts with the coating film surface, the penetration speed of water in the direction of film thickness is controlled since the metal hydroxide clusters exist in the film. Therefore, when water flows away, only the film in a water penetrated portion in the vicinity of the coating film surface flows and the entire coating film does not vanish at one time.