To prevent occurrence of vibration at a knife-edge by constituting a cutting blade portion so that a face angle surface has a very narrow angle and a flank angle surface has the narrowest angle nearest an outer diameter line.

In a cutting blade portion comprising a first face angle surface 5 with a slitting size (b) and a first flank angle surface 7 extending by a length longer than the slitting size (b), the first face angle surface 5 has a very narrow angle and the first flank angle surface 7 has the narrowest angle nearest an outer diameter line 8. Because a knife-edge is not sharp and is reinforced, the knife-edge is fast, and does not go into a machined material. Because a cutting peripheral surface abuts to the first flank angle surface during cutting of the machined material, occurrence of vibration at a knife-edge is prevented to avoid defect accident caused by it, and a second flank angle surface 8 reduces cutting resistance. Because the first face angle surface 7 has the slitting size (b) and its sequential second face angle surface 8 is wide angled, chip discharge becomes good.