1. An image processing apparatus, comprising: a device which recognizes, in a bone part unit, a bone area extracted from a medical image and including a bone region constituted of several bone parts; a device which is used to select a bone region to be displayed; a device which is used to determine the bone parts corresponding to the selected bone region; a device which is used to determine a viewpoint and a sight line direction for observing the selected bone region; a device which generates a volume rendering image which displays the bone area with the viewpoint and the sight line direction, based on the bone area of the medical image recognized in the bone part unit and the determined viewpoint and sight line direction; and a device which is used to conduct control to display the generated volume rendering image.