A safe and efficient machining process design is realized by extracting the unmachined shape and by three-dimensionally visualizing the unmachined shape without generating a machining track. The present device 1 comprises a displacement unit 2 for displacing a machining tool on the surface of a three-dimensional product shape model at arbitrary positions without causing interference between the product shape model and the machining tool, a detection unit 3 for detecting a distance between the arbitrary position and the machining tool, and a preparation unit 4 for preparing the three-dimensional unmachined shape by using the minimum value of the distance obtained for each point on the surface of the shape model when the arbitrary point is moved.