A joint for the connection of a secondary duct to a main duct provided with a cutout having predefined shape and dimensions, said secondary duct being inclined with respect to said main duct, the joint comprises - a tubular element provided with a first thread on a first external portion thereof and with a second thread on a second internal portion thereof; - at least one ring provided with an internal thread which is complementary to said first external thread of said tubular element, said ring allowing its screwing onto said first external thread of said tubular element; - at least one first shell whose shape and dimensions are adapted to be complementary and exceed those of said cutout for its easy insertion within said main duct through said cutout and its arrangement so as to overlap said cutout, with consequent complete occlusion of said cutout, with a perimetric border thereof adapted to be in abutment against the internal surface of said main duct at the edge of said cutout, said first shell comprising an inclined tubular channel provided with an external thread which is complementary to said second thread of said tubular element for the coupling of said element to said channel following mutual screw coupling; - at least one second shell whose shape is adapted to be substantially complementary to the shape of the external surface of said duct and whose dimensions exceed the dimensions of said cutout, said second shell being provided with an inclined tubular body suitable for the accommodation of said tubular element and of said channel; the clamping of said ring on said first thread, with said ring in abutment against the terminal front of said inclined tubular body of said second shell, causing in use the clamping of the edge of said cutout between corresponding perimetric borders (10, 15) of said first shell and of said second shell. - a tubular element provided with a first thread on a first external portion thereof and with a second thread on a second internal portion thereof; - at least one ring provided with an internal thread which is complementary to said first external thread of said tubular element, said ring allowing its screwing onto said first external thread of said tubular element; - at least one first shell whose shape and dimensions are adapted to be complementary and exceed those of said cutout for its easy insertion within said main duct through said cutout and its arrangement so as to overlap said cutout, with consequent complete occlusion of said cutout, with a perimetric border thereof adapted to be in abutment against the internal surface of said main duct at the edge of said cutout, said first shell comprising an inclined tubular channel provided with an external thread which is complementary to said second thread of said tubular element for the coupling of said element to said channel following mutual screw coupling; - at least one second shell whose shape is adapted to be substantially complementary to the shape of the external surface of said duct and whose dimensions exceed the dimensions of said cutout, said second shell being provided with an inclined tubular body suitable for the accommodation of said tubular element and of said channel;