To improve durability by preventing generation of chipping and to provide excellent working accuracy.

Generation of very fine chipping of a cutting edge tip is suppressed to improve durability since a circular-arc shaped margin 32 having the same diameter as an outer peripheral edge 22 by a width dimension (margin width tm) of 0.01 mm to 0.02 mm in the outer peripheral edge 22 made of ultra- high pressure CBN sintered body 28. Excellent roughness of a working surface is obtained by maintaining sharpness of the same degree as that in the case of no-margin since the margin width tm of 0.01 mm to 0.02 mm is very narrow. When relief grinding is performed for a first flank 34 succeeding to the margin 32, the diameter dimension of the cutting edge tip is maintained constant, even if a boundary position between the cutting edge tip and the margin 32 is slightly deviated, relief grinding is facilitated and dimensional accuracy of working by a square end mill 10 is improved.