The invention provides a device for fastening a fastening member for positioning and fastening a hexagon bolt (2) to a nut (7) having an opening part (7a). The device comprises a rotating shaft body (1) rotated by a drive source, a freely advancing and retreating socket (3) connected to the tip of the rotating shaft body (1) and to which the hexagon bolt (2) is fitted, and a guide mechanism (5) formed of three claw-like members (4) operably operated according to the advance and retreat operation of the socket (3). The hexagon bolt (2) fitted to the socket (3) by inserting the tip part of the guide mechanism (5) in a closed state into the opening part (7a) of the nut (7) is positioned and fastened to the nut (7).