An end mill having a plurality of cutting edges on a surface of a rod-shaped main body is rotated about its longitudinal axis and is moved relative to a workpiece in a feed direction tangential and orthogonal to the longitudinal axis along the workpiece. The end mill is also moved relative to the workpiece in a reciprocal manner in the axial direction while it is moved in the feed direction. In this way, a plurality of dimples are formed by the end mill on the workpiece, such that the dimples are relatively spaced apart from each other on the workpiece. The plurality of dimples are arranged side by side in the feed direction along a line inclined at a predetermined angle relative to the feed direction so as to be offset while partially overlapping with respect to each other in the axial direction.