PURPOSE: To make it possible either to gradually expand the passing area of chips toward the shank section or to enlarge the back taper by gradually enlarging the radius of carvature of the spiral groove formed on the external peripheral surface of a drill with getting near the vicinity of the shank section.

CONSTITUTION: A spiral groove 4 on the external peripheral surface of a drill 3 is formed so as to become large as the radius of curvature R of the groove gets near a shank section 3b. That is, when R1, R2, and R3 stand for the respective radius of curvatures in the vicinity of an end section 3a, a middle section 3c, and the shank section 3b, the spiral groove 4 is formed with the radius of curvature R continuously varied with such relation as R1<R2<R3. When the passing area of the chips produced by the spiral groove 4 is fixed by thiis constitution, the back taper can be formed large to improve the rigidity maintaining the effective discharge of chips, machining accuracy, and the strength of the drill. When the back taper is fixed, the passing area of chips can be expanded improving the chip discharge effect.