PURPOSE: To enable the transmission of high torque by inclining a recessed portion inward toward a bottom face, giving bite wall surfaces in a conical face extended to the bottom face, and forming plural radial grooves extending outward from wall faces in parallel to the axis.
CONSTITUTION: When inserting a driver-bit 20 is inserted into a processed portion 12 of a screw 10, both the two produce mutual bite action because curved wall surfaces 16 constituting part of a conical face C and joining surfaces 25 touches one another. There is a fitting--clearance gap between side wall surfaces 14a, 14b of an angular groove 13 and side surfaces 24a, 24b of an inpeller portion 23. When torque is applied to the driver-bit 20 in this condition, the torque is transmitted to a screw 10 due to frictional force in the bite action. When the torque exceeds the frictional force, the bit 20 somewhat turns in the screw 10, so that the outer part of the side surface 24a touches the side wall surface 14a, thereby transmitting a specified screwing-in torque to the screw 10. Accordingly, the torque is transmitted without comeout of the drive-bit 20.