PURPOSE: To perfectly eliminate the chip jamming and stabilize the life of a tool by forming the outer peripheral cutting edge and the end cutting edge of a cutter part which is continuous to a shank part, to a continuous blade form and forming a gap in 10-90% of the cutter diameter at the core part of the cutter part.

CONSTITUTION: The ridge line of an outer peripheral cutting edge is formed to a staircase form, and each top point is inscribed in a supposed conical shape having a prescribed tapered angle, and the ridge line 2 directed towards a shank from each top point has an angle in a range over 0° and less than 90° towards the tool axis, for the tool axis line. Accordingly, the discharge performance for chips can be improved, and high speed cutting is enabled.