In this insert, in addition to the insert being formed so that the cutting edge is twisted at a helix angle within the range of 5° to 25° about the axis when the insert is attached to the insert mounting seat, the thickness of the insert body at the flank located on a straight line, which passes through the center of the rough hemisphere formed by the rotational locus of the cutting edge and forms an angle of inclination of 90° relative to the axis, is set to be within the range of 0.5D to 0.9D relative to the thickness D of the rough plate that composes the insert body. As a result, since a comparatively gentle but adequately large helix angle is imparted to the cutting edge, cutting resistance decreases. In addition, since the thickness of the insert body at the flank on the rear end side of the insert body is not excessively thin, the rigidity of the insert body is adequately secured.