1. A three dimensional image processing apparatus comprising: a storage unit which stores data of a plurality of images in different radiographing directions together with information concerning cardiac phases; a key image selection unit which selects a plurality of key images from said plurality of images; a feature point designation unit which designates feature points on the selected key images in accordance with operation by an operator; and an image reconstruction unit which reconstructs a three dimensional image from said plurality of images on the basis of positions of the designated feature points, wherein the key image selection unit selects, as the key images, a pair of images which are located at the same cardiac phase and whose radiographing directions are different by a substantially predetermined angle and a plurality of images spaced apart from each other by an angle obtained by substantially equally dividing an interval between the pair of images by a predetermined number.