PURPOSE: To surely guide and insert a driver bit into a screw slot so as to fasten/loosen a screw efficiently by arranging a guide projection on the tip face of the driver bit.

CONSTITUTION: A screw 16 and a driver bit 10 are set on the same axial line, and when the driver bit 10 is advance toward the screw 16 while rotating, a guide projection 12 arranged on the tip face of the driver bit 10 catches a slot 16a of the screw 16. In this way, the tip part of the drive bit 10 is guided to be inserted into the slot 16a. In this process, the driver bit 10 can be surely guided and inserted into the slot 16a even when the slot 16a of the screw 16 is formed eccentrically form the axial line of the screw 16 because the guide projection 12 is arranged eccentrically from the axial line of the driver bit 10. Consequently, the screw 16 can be efficiently fastened/loosened by means of the driver bit 10.