1. A machining program preparation apparatus that prepares a machining program for a machining center that executes milling and turning, the machining program preparation apparatus comprising: a machining region extracting unit that extracts a machining region based on a given three-dimensional part shape and a given three-dimensional material shape; an envelope shape generating unit that generates a three-dimensional envelope shape that envelopes the given three-dimensional part shape based on this part shape; a turning region extracting unit that extracts a three-dimensional turning region from the material shape and the envelope shape; and a milling region extracting unit that extracts a three-dimensional milling region from the machining region extracted by the machining region extracting unit and the turning region extracted by the turning region extracting unit.