A machining method for forming an opening through a workpiece, serving as a green compact, by moving a milling tool in a radial direction relative to the workpiece is provided. The milling tool includes a first milling tool and a second milling tool. The method includes a pre-machining step of using the first milling tool, and forming the first sidewall surface by rotating the first milling tool so as to cause cutting edges of the first milling tool to cut the workpiece from the acute angle corner to the obtuse angle corner, while leaving a cutting allowance on the second sidewall surface; and a post-machining step of using the second milling tool whose cutting edges are reversed from the cutting edges of the first milling tool, and forming the second sidewall surface by rotating the second milling tool so as to cause the cutting edges of the second milling tool to cut the cutting allowance from the acute angle corner to the obtuse angle corner.