To effectively suppress chattering vibration of cutting edges to improve the durability and to improve the hole surface cutting accuracy.

Both tips 17, 18 fixed to the tip part 15 of a shank 12 are provided at symmetrical positions in diametrically opposite directions. Further, the respective cutting edgees 19, 20 of both the tips are provided with the first cutting edge parts 19a, 20a for cutting small diameter tapered surfaces, at the tip parts, the second cutting edge parts 19b, 20b which are continuously formed at the end edges of the respective first cutting edge parts and which cut larger taper angles, and the third cutting edge parts 19c, 20c which are provided at the end edges of the respective cutting edge parts, and which are used for cutting holes having the largest diameter larger than the diameter of the shank 12. Furthermore, a pair of guide pads 21, 22 are provided at the positions circumferentially 90 degrees turned from the positions of the respective tips, on the shaft part 15a of the tip part of the shank.