PURPOSE: To provide a protective layer forming agent for a processed paper, having excellent performance in providing oil resistance, plasticizer resistance, water resistance and preservability and having excellent application properties, by using an aqueous solution of a graft copolymer having a specified composition and a cross-linking agent having a specified functional group as main constituents.

CONSTITUTION: In the composition of the graft copolymer, polyvinyl alcohol or a copolymerizable denatured polyvinyl alcohol is used as a backbone polymer, whereby wetting properties for a covered paper and adhesion are enhanced, and oil resistance and plasticizer resistance are provided. Acrylamide or methacrylamide is used as a comonomer with the backbone polymer, thereby providing water-soluble nature and an ability of preventing foaming of the aqueous solution due to the backbone polymer. Further, a polymerizable monomer having an acetoacetyl group is copolymerized with the copolymer, thereby providing a high reactivity to a cross-linking agent, and the cross-linking agent having a functional group such as aldehyde group and methylol group is brought into reaction with the copolymer, thereby effecting cross-linking with the graft copolymer to form a rigid three-dimensional meshed structure. The coated film layer thus provided augment the capability of preventing penetration of an oil, a plasticizer, water or the like, and further enhances the performance as the protective layer.