A method whereby the numerical control information preparing function determines the manner of machining. On the basis of the input shape of a workpiece and on the basis of the input shape of a component part to be obtained from the workpiece, a portion of the workpiece which is to be subjected to outer diameter machining and a portion of the same which is to be subjected to inner diameter machining are determined. Thereafter, a division candidate point is obtained so that each of the portions may be divided, at the division candidate point, into a sub-portion to be subjected to forward cutting and a sub-portion to be subjected to backward cutting. On the basis of the configuration of the backward cutting sub-portion and the shape of the tool to be used, a check is made as to whether any part of the sub-portion can remain uncut after the backward cutting. On the basis of the result of this check, a determination is made as to whether the entire portion should be divided at the division candidate point. If the division is determined, the portion is divided into the forward cutting sub-portion and the backward cutting sub-portion, with the division candidate point serving as the boundary. Thus, each of the outer diameter machining portion and the inner diameter machining portion can be divided into a forward cutting sub-portion and a backward cutting sub-portion at the optimum point.