PURPOSE: To improve workability and to provide a finish product having excellent size accuracy and surface characteristics by feeding a diamond as a diamond head is pressed against the turned surfaces of a metallic material in a state to rotate the metallic material after a turning work, and squeezing the tool marks of turned surfaces.

CONSTITUTION: By using a CNC lathe having a tool 51 to the tip of which a diamond 61 is fixed, a metallic material 11 after a turning work is rotated. With this state, the diamond 61 is fed as the diamond 61 is pressed against turned surfaces 21, 22, and 23 of the metallic material 11 to squeeze the tool marks of the turned surfaces 21, 22, and 23.