Plastics are indispensable for our daily lives. However, since their excess use has caused serious environmental pollution, alternative materials are highly desired to realize a sustainable society. Paper is a natural material made of cellulose fibers from abundant resources such as grasses and trees, but it cannot replace plastics, particularly because of its permeability to water. Here, we report an unprecedented coating technique that allows paper to be used as a plastic. By dipping a paper or a paper product into a low-viscosity liquid mainly composed of an alkoxysilane and then drying under ambient conditions, a porous silica-resin layer of a few micrometers of thickness, which provides the paper with excellent durability including water resistance without losing its original texture and flexibility, is uniformly formed on the cellulose fibers. The reaction is basically the same as in the conventional sol-gel method but does not need large amounts of water and acid or alkali catalyst, which would degrade the substrate paper, in this advanced method using an accelerant. The thus-obtained organic-inorganic hybrid materials can replace many plastic products and are easily decomposed in the natural environment without causing pollution.