Abstract: The device comprises, inside a case, a tightening cam (7) hinged about an axis (8) such that, unused, it does not engage a belaying rope (2) passing through the case and, when the device is subjected to a set acceleration downwards, it pivots towards the rope (2) to tighten it to the case with self-tightening effect. On the cam (7) is mounted a detecting roller (10) which co-operates with the rope (2) when the cam (7) is unused. A spring actuated (13) friction piece (12), at a certain rotational speed of the roller (10), subjects it to a resisting moment which triggers off the pivoting of the cam (7) in the tightening direction. A lever (15), to which the user is linked, engages in a unidirectional manner the cam (7) to push it in tightening position if the user falls. The device simultaneously triggers off several tightening effects in case of a fall.