To preserve a woody material which is made fragile and hollow without changing the hue and appearance of the material by penetrating the alcohol solution of polyvinyl butyral into the material.

The alcohol solution of polyvinyl butyral or it and ethyl cellulose is penetrated into a woody material which is made fragile or hollow. In other words, the entire material is immersed directly into the solution, or the solution is injected into or applied on the required part of the material, and the material is dried and cured. The solution can penetrate the material sufficiently in a very short time. The drying is preferably done at room temperature until the weight becomes constant.