Method for preparing NC machining program and apparatus for preparing NC machining program

A preparing apparatus of an NC machining program prepares an NC machining program including a position instruction of a cutting portion of a rotary tool on the basis of a two-dimensional workpiece coordinate system having a Z-axis and an X-axis as coordinate axes. An operator inputs a lead clearance α and a radial clearance β to the preparing apparatus via an input operation section. A processing section converts the NC machining program prepared on the basis of the two-dimensional workpiece coordinate system into an NC machining program including a position instruction of a three-dimensional workpiece coordinate system having the X-axis, a Y-axis and the Z-axis as the coordinate axes, on the basis of the lead clearance α and the radial clearance β.