To prevent decrease in roundness due to actual working, particularly, in working precision at an approaching part and a releasing part by finishing the working by an up-cutting work such that the relative motion direction of a work and a tool is reverse to the rotating direction of the tool when the work is made apart from the tool.

A work 21 is set to a table of a machine tool, and a tool 20 is rotated clockwise. Next, with respect to the relative position of the work 21 and the tool 20, the work 21 and the tool 20 are moved to a height position (a) to enable a circular arc to be worked promptly in the tangential line direction of the circular arc to be desired upon finishing the circular arc working. Further, the table is moved until the tool 20 reaches a position b as an approaching operation. After that, the table is moved so that a start point and an end point are set to the position (b) and a circular arc interpolation is executed by diameter of the circular arc to be finished. Finally, the table is moved until the tool 20 reaches a position (c) as a releasing operation. As described above, in the case of circular arc working, the working precision upon up-cutting work is always improved.