The invention concerns an adjustable fixing device for straightening and supporting the bone system, in particular the back bone, comprising at least an anchoring element (1) with at least one part (5) capable of being fixed relative to the bone, a head (3) with an axis (XX'), an intermediate ring (9), and a blocking element (6) capable of being fixed on the head (3) end (7). The invention is characterised in that the surface (61) pressing the blocking element (6) against a clamping member (8) for locking in an orifice (10) lateral relative to the axis (XX') a rod connecting (4) several fixing devices, is hollow and spherical and the zone pressing (91) the intermediate ring against this clamping member (8) is an internal circular ridge; and said clamping member (8) consists of at least two separable parts (81, 82) each comprising a half-collar (121, 122) forming together said clamping orifice (10) and a part forming a bore (11) flange (131, 132) adapted to be threaded on said head (3) and whereof the external pressing surface (141, 142) has a spherical shape matching with the intermediate ring corresponding (9) pressing shapes and of the blocking element (6).