PURPOSE: To intensify the strength of a cutting edge without increasing the horsepower required for cutting by providing a surface of a primary angle in which the degree of acute angle is reduced.

CONSTITUTION: The angle 30 of the surface 27 of a primary angle is made to be an arbitrary angle from the negative 90 degrees to a positive angle 29 (=θ) smaller than the clearance angle 28 of the principal cutting face 17, and preferably to be within the range of 0 degree to positive 5 degrees. Further, the extended width of this primary angle surface 27 from the cutting edge is made to be equal to or smaller than the maximum design cutting depth of the cutting edge. As a result, the flow of chips from a workpiece to the principal cutting face 17 having an inclined surface of the positive angle 28 (preferably 17 to 20 degrees) is not prevented, and also the cutting edge of a lay-down type cutter insert 11 consisting of ceramic, silicon nitride, or similar material is strengthened.