To suppress a shock and vibration when cutting a corner portion of a rectangular workpiece and forming the workpiece into a cylindrical shaft shape by the construction such that at least one of a plurality of cutters may cut the workpiece.

Cutters 6a to 6c are assembled to an edge of a supporting wall 52 of a holder 5. Upon assembling, the cutters 6a to 6c are arranged in a manner such that the outermost position in the radial direction of corner portions 10a to 10d of a workpiece 1 is cut by the cutter 6a, the intermediate position in the radial direction of the corner portions 10a to 10d is cut by the cutter 6b, and the innermost position in the radial direction of the corner portions 10a to 10d is cut by the cutter 6c in association with the rotation of the workpiece. The cutting of the first, corner portion 10a is started by the cutter 6b before finishing the cutting of the first corner portion 10a by the cutter 6a. Subsequently, the cutting of the first corner portion 10a is started by the cutter 6c. The cutting of the next corner portion 10b is started by the cutter 6a before finishing the cutting of the first corner portion 10a by the cutter 6c. As mentioned above, the corner portions 10a to 10d are sequentially cut.