The device comprises a sleeve (14) which can be associated with the rigid tube or union, and at least one ring (15) which can be associated with the flexible tube, a threaded coupling (16) on the sleeve and the ring to mutually connect them and force them axially against each other, a series of annular protrusions (17) which are associated with the ring and protrude radially toward the axis of the ring to engage respective grooves (12) which separate corresponding undulations (11) of the flexible tube and to push the undulated flexible tube into forced contact engagement against the sleeve when the threaded coupling is tightened by turning the ring (15) with respect to the sleeve (14); the annular protrusions (17) associated with the ring have a profile which is complementary to the profile of the grooves which separate the undulations of the flexible tube.