A method of preparing an NC part program for numerically-controlled machining includes a plurality of machining steps, e.g., a first machining step for machining workpiece (WK) into a roughly-machined shape (P5 - P4 - P3 - P2 - P1), and a second machining step for machining the workpiece into a finished shape (P6 - P7 - P8 - P9 - P10). The method comprises: inputting, for each machining step, at least the tool to be used TL1, TL2, the axial incremental quantity between a reference position (R) on a tool slide (TBS) and the end of the tool used, and positions (P5 to P1; P6 to P10) to be machined; preparing a subprogram for each machining step by employing these items of data; preparing a main program for calling the subprograms in the order of machining step; and combining the subprograms and the main program to prepare an NC part program.