Abstract: The present invention is related to a fastening buckle intended for fastening safety straps, especially webbing of mountaineering and climbing harnesses and/or harnesses for securing persons working in height. The buckle comprises an upper frame (1) and a lower frame (2). One of the lateral sides (5) of the upper frame (1) is interrupted by a gap (6) having greater width than the thickness of the fastened strap (3). For preventing the possibility of cutting the fastened strap (3) in the transversal slot (14) between lower frame (2) and upper frame (1) at dynamic load, it is useful to bend the second long side (10) of the lower frame (2) at a sharp angle (a) with respect to the plane of the first long side (8) of the lower frame (2).