PURPOSE: To perform an efficient adhesion of a single or a plural number of plastic sheets, by a method wherein a flexible thin pressurizing piece is secured to the shape of an adhering surface of a shet on a flexible thin laminar base, and a template, in which both or either of the base and the pressurizing piece produce an electrode or a thermal conductive material, is used.

CONSTITUTION: An flexible thin pressurizing piece 3 is secured to the shape of an adhering surface of a sheet or a flexible thin laminar base to form a templat. Both of the base 2 and the pressurizing piece 3 form an electric or a thermal conductive material, or the one of them forms an electric or a thermal conductive material and the other forms a non-conductive material. The manufacture of a polymer in the shape of a bag with a gusset is such that, in a unit 5 in which 3 sides of an oblong sheet are folded inward to form ausset-forming segments 6, the template 1 is inserted between the body and the gusset-forming segment 6, the unit 5 is placed on a platen 7, a unit 5', into which no template 1 is inserted, is placed thereon, the template 1 and the platen 7 produce the same pole, and an electrode mold 8, producing a different pole, is depressed to apply a high-frequency voltage.