A bone fixing device, in particular for fixing to the sacrum for osteosynthesis of the backbone, comprises elongate link means receiving at least one bone-fastening screw, which passes through an orifice formed in the link means. In the bottom of the link means there is included a bearing surface of essentially circular cross-section. The head of the screw includes an essentially spherical surface for bearing against said bearing surface. The link means include a first thread in the vicinity of said orifice. The device further includes a plug having a second thread suitable for co-operating with the first thread, the plug being suitable for coming into clamping contact against said screw head to hold it in a desired angular position. According to the invention, the link means are constituted by a single-piece plate-shaped element having the orifice and the first thread formed therein, and said bearing surface is essentially spherical.