PURPOSE: To firmly install a tip to a tip mounting seat and enhance the mounting rigidity by forming a protruding part on one of a sitting surface and a receiving surface on the tip mounting seat side, and a recessed groove part to be fitted to the protruding part on the other.

CONSTITUTION: The protruding part 17 of the mounting seat bottom surface 14A of a tip mounting seat 14 and the recessed groove part 22 of a throw away tip 15 are formed in the direction orthogonal to the thickness direction of a tool body 11 along the tool rotating direction, or in the plane direction orthogonal to the axial line direction of the tool body 11. The recessed groove part 22 is fitted to the protruding part 17, whereby the tip 15 is precisely positioned in the axial line direction. Since the cutting edge 21 formed on the tip 15 is arranged in parallel to the axial line of the tool body 11, the run out of each cutting edge 21 provided on the tool body 11 can be fitted with high precision.