A process planning apparatus extracts a region to be machined based on the difference of the shape data before and after machining of the workpiece, replaces the extracted region into combinations of the predetermined machining features, allocates a predetermined fixed cycle to each of the replaced machining features, and applies an assessment function relating to a machining time and a life of the end mill to each of the combinations of the machining features to which the fixed cycles are respectively allocated, thereby selecting a group of the fixed cycles which makes an assessment value obtained by the assessment function optimum as the optimal process. By these steps, it becomes possible to design the process for causing the NC machine employing end mills as a cutting tool to perform a predetermined machining of a workpiece without relying on the experience of the designer, and without necessitating complicated work.