PURPOSE: To improve the peeling and scaling off of a formed polymer layer without detriment to the ground film, by applying a polar group-carrying monomer containing a specified polymerization initiator to the surface of a nonpolar plastic film, and polymerizing the monomer.

CONSTITUTION: A composition prepared by mixing a polar groupcarrying monomer with a polymerization initiator comprising a mixture containing (i) a dialkyl peroxide and/or a peroxy ketal and (ii) a diacyl peroxide and/or a peroxy ester is applied to at least one of the surfaces of a film or sheet of a nonpolar plastic such as polyethylene or polypropylene. Then, this layer applied is polymerized by heating, whereby a polar group-carrying layer of a thickness of below 1μm is formed on at least one of the surface of the film or sheet. The amount of the polymerization initiator used is preferably 0.2W5.0pts.wt. per 100pts.wt. monomer.