A clamping screw (1) has a shaft (3) with a thread and a rectangular screw head (2) which can be form-fittingly inserted into a T-groove (4) of an element (25). In the mounting position, the screw head (2) extends transversely to a longitudinal slot (5) of the T-groove (4) and has clamping sections (7) which engage from behind supporting branches (6) on both sides of the longitudinal slot (5). The clamping sections comprise each a straight edge (12) and a blocking edge (13) that can be supported against a wall (14) of the groove. The blocking edge (13) projects out of a side surface which is part of a longitudinal side (11). A straight line (15) connecting the straight edge (12) and the blocking edge (13) of the same longitudinal side (11) crosses a transverse axis (10) of the screw head (2) at an angle other than a 90 DEG angle.