To improve the finished surface roughness by polishing the tip part of a diamond film coated on the end cutting edge of a diamond coating end mill by the gas phase composite method with the plane perpendicular to the axis of an end mill shaft.

A groove side surface of a workpiece is worked with an outer peripheral cutting edge 5, and the bottom of the groove of the workpiece is worked with an end cutting edge 7. A part of a diamond film coated on a part of the corner 6 of the end cutting edge 7 is polished with the plane perpendicular to the axis of an end mill shaft. The dimensions are such that when the diameter is about 10mm, a triangular or square smooth surface 9 of about 0.1-0.3mm from the corner toward the shaft center and about 0.01-0.1mm to the bottom rake angle direction is formed. Of course, the periphery of a smooth surface 9 is coated with diamond crystal particles 11, and the smooth surface 9 is also diamond-coated. In the case of groove cutting with the diamond coating square endmill 1 in which the corner 6 of the end cutting edge 7 is ground, the finished roughness of the bottom is improved.