Abstract: Screw fixing device for a concrete construction element comprising a screw (1) and a sheath (4) of synthetic material intended to be fixed in the concrete. The sheath and the screw possess a trapezoidal thread. Externally, the sheath possesses peripheral anchoring ribs (7) of triangular section, except in its upper part (6). The slope of the flanks (8) of the ribs facing the entry to the sheath is about 30.degree.. The sheath of synthetic material is surrounded by a metal sheath (12) bearing on these flanks (8) but having a play relative to the other flanks (9) of the ribs.