PURPOSE: To suppress the blister and release of a waterproof layer, by coating a substrate with a solution of polyvinyl butyral resin to form a film having improved waterproofness, followed by applying an emulsion type waterproofing agent to the film.

CONSTITUTION: A resin solution consisting of (A) polyvinyl butyral resin, (B) solvent (preferably an alcohol, e.g., isopropanol, etc.), and (C) plasticizer (e.g., dibutyl phthalate, etc.), having a brittle point ≤-10°C and a viscosity preferably ≤30,000cps is applied to a substrate uniformly to form a film with an application amount preferably 0.2W1kg/m2, and the film is dried so that the substrate is completely covered with the thin waterproof film. A waterproofing agent of the emulsion type film (e.g., rubber asphalt emulsion, etc.) in an application amount 1W5kg/m2 is applied to the film. Preferably the amount of the component A is 4W23wt%, the amount of the component B is 95W75wt% and the amount of the component C is 1W12wt%.