PURPOSE: To improve the airtightness, adherence and durability of an insert portion, by using an insert material made by applying a film on the surface of said insert material at the time of insert injection molding of thermoplastic resin, said film being a coating agent made by mixing a specified compound with an organic polymeric compound.

CONSTITUTION: In an insert injection molding of thermoplastic resin, the presence of an organic polymeric compound is indispensable as a component of the film to be applied onto the surface of insert material, and such material is preferably polyurethane resin, polyester resin, polyamide resin, elastomer, synthetic rubber or others. Further, in order to improve the airtightness of insert molding, such organic polymeric compound is added and mixed with one or a plurality of kinds of compound selected from spirorthoester, spirorthocarbonate, trioxyacycloctane and ketallactone compounds.