PURPOSE: To enable the state of no clearance to a drill through simple work by setting a radius of curvature of an inner surface of cutting part, extended in the axial front along the flank of the drill from a body part, to almost the same radius of a curvature as the flank.

CONSTITUTION: A chamfering cutter 2 is provided with a ringlike body part 8 externally fittable to a drill 1, while the radius of an inner circumferential surface 8a of the body 8 is set to be almost the same as a radius of curvature of a land 7 of the drill 1, and a pair of tapped holes 9 are pierced through the body part 8, and each set screw 10 is screwed thereinto. in response to a flank 6 of the drill 1, a pair of cutting parts 11 are extended in a spiral state from each lower position of these tapped holes 9, while a conical oblique curved surface 12 is formed at the lower end, and a front edge of this oblique curved surface 12 in the cutting rotation direction X of the drill 1 comes to a cutting edge 13. A radius of curvature of each inner surface 11b of these cutting parts 11 is set to be almost the same as that of the flank 6 at every part. When the chamfering cutter 2 is clamped to the drill 1, each inner surface 11b of both these cutting parts 11 comes into contact with the flank 6 whereby there is a state of no clearance between them.