Apparatus for treating surfaces of elongated or endless elements made of plastic material and moving along an advancing direction, comprising: - a treatment chamber provided with an inlet opening and an outlet opening that are aligned along said advancing direction to allow an elongated element made of plastic material to move through said treatment chamber; - suction means connected to said treatment chamber to create therein a definite first operating pressure (Pal) that is lower than atmospheric pressure; - inlet means for inletting at least one gas (G1, G2) inside said treatment chamber; - an emitter placed inside said treatment chamber and powered by a generator in order to generate inside said treatment chamber an electric field adapted to form with at least said gas (G1, G2) a plasma able for interacting with an external surface of said elongated element, in particular polarizing molecules of polymeric chains of said plastic material and/or forming oxidizing radicals, so as to increase a surface tension (Ts) of said external surface up to a defined minimum value (Ts min); - at least one first sealing gasket and at least one second sealing gasket arranged at said outlet opening and said inlet opening respectively and shaped such as to abut, in particular with interference, said external surface of said elongated element in order to prevent air from entering into said treatment chamber, said at least one first sealing gasket being made of compact elastic polyurethane with hardness between 55 and 75 Shore A, in particular between 60 and 65 Shore configured to not interact and modify the surface tension (Ts) of said external surface obtained in said treatment chamber. - a treatment chamber provided with an inlet opening and an outlet opening that are aligned along said advancing direction to allow an elongated element made of plastic material to move through said treatment chamber; - suction means connected to said treatment chamber to create therein a definite first operating pressure (Pal) that is lower than atmospheric pressure; - inlet means for inletting at least one gas (G1, G2) inside said treatment chamber; - an emitter placed inside said treatment chamber and powered by a generator in order to generate inside said treatment chamber an electric field adapted to form with at least said gas (G1, G2) a plasma able for interacting with an external surface of said elongated element, in particular polarizing molecules of polymeric chains of said plastic material and/or forming oxidizing radicals, so as to increase a surface tension (Ts) of said external surface up to a defined minimum value (Ts min); - at least one first sealing gasket and at least one second sealing gasket arranged at said outlet opening and said inlet opening respectively and shaped such as to abut, in particular with interference, said external surface of said elongated element in order to prevent air from entering into said treatment chamber, said at least one first sealing gasket being made of compact elastic polyurethane with hardness between 55 and 75 Shore A, in particular between 60 and 65 Shore configured to not interact and modify the surface tension (Ts) of said external surface obtained in said treatment chamber.