To cut and separate a raw material into a semi-spherical shape by machining capable of omitting complicated steps and high-level facilities.

A center portion of a workpiece mounted to a lathe is cut and opened, so as to secure a turning center of a cutting tool. Then, a cutting tool 19 is installed, so as to devise a cutting blade holding arm concentrically turning in a direction perpendicular to a rotary shaft of the workpiece 14 to make the turning center of the cutting tool is formed into a semi-spherical shape, and with a sphere identical to a cutting spherical groove made by the cutting blade during cutting. The cutting blade holding arm has a sectional area larger than that of the cutting blade and sufficient strength, and the cutting blade can be transferred to the rotary shaft of the workpiece smoothly, so as to cut and separate the workpiece.