Abstract: An assembly device (1) of a first element (4) on a tubular part (3) equipped with a fluid channel comprises a second element (9) slidingly mounted with respect to said first element (4). A first axial through passage (35)is arranged in the second element (9), and coupling arms (26) securedly unite the first element (4, 37) to the part (3) in a predetermined axial position, in which the second element (9) presses axially around an opening of the fluid channel of said part (3), allowing communication of the channel with said first axial passage (35). A first seal (16) is fitted between the second element (9) and the part (3), and an internal space (19) extends in the axial direction between opposite faces (6, 18) respectively of the first and second elements (4, 9), said space being in communication with the first axial passage (35) of the second element (9), in such a way that the fluid pressure in the space (19) urges the second element (9) against the part (3).