PURPOSE: To improve a cutting capacity by forming the rake angles in the axial and radial directions of a tip mounting seat face at 15°W20° and providing a clearance angle which is slightly larger than said rake angle on the cutting edge of a square truncated cone shaped tool tip.

CONSTITUTION: A tip mounting seat face 5 which is formed into nearly a square shaped flat face is formed on the side facing a chip pocket 4 of the end part of each tip holding part 3. A relatively large rake angle β is provided in the axial direction on each tip seat face 5 and its angle is in the range of 15°W20°. A tool tip 10 which is mounted on the tip mounting seat face 5 is of a throw away type and has a square truncated cone shape while its cutting edge is provided on the four sides of its bottom face side outer periphery. The inclination angle of an inclined side face which forms the flank of the cutting edge is slightly larger than that of the rake angle with respect to a center line. Accordingly, the chips produced by the square cutting edge can be made thick improving a cutting capacity.