1. An image processing apparatus comprising: an image obtaining unit that obtains an X-ray projection image of a subject; a positioning-image creating unit connected to the image obtaining unit for creating positioning images with a subject center being changed within a certain range, under a condition that a point of view and a projection plane are fixed, for each of a plurality of different projection conditions using volume data collected from the subject by a medical imaging apparatus; a projection-condition obtaining unit connected to the positioning-image creating unit for selecting an image most analogous to the X-ray projection image of the subject from the positioning images, and obtains a projection condition of the selected positioning image; a target-region storage unit that stores volume data of a target region, among the volume data collected, as target-region volume data; and an image display unit connected to the target-region storage unit and the projection-condition obtaining unit for creating and displaying an image of the target region based on the target-region volume data stored in the target-region storage unit and the projection condition obtained by the projection-condition obtaining unit.