PURPOSE: To enhance the machining efficiency without impairing the rigidity of a tool by forming polygonal the section perpendicular to the tool axis and providing a channel-form cutter groove along the ridge of a cutting edge, wherein a superhard alloy is used to the tool material.

CONSTITUTION: The tool material of an end mill concerned consists of a superhard alloy, and its section perpendicular to the tool axis is formed polygonal. A channel-form cutter groove 3 is provided having a size less than one half of the length of one edge member of the polygon along the ridge of the cutting edge so that the rigidities of the cutting edge 2 and tool are maintained, and small splash generated at machining is accommodated temporarily. To suppress burrs generated at the oversurface of a work to be processed at machining, the twist of an outside circumferential cutting edge 2 is oriented leftwise, and the twist angle is ranged between 5-20 deg., preferably between 10-15 deg., where the influences upon the machining resistance and the accuracy of the surface to be processed are taken into consideration.