1. An X-ray imaging apparatus comprising: an imaging unit that irradiates X-rays on a subject and detects X-rays penetrating the subject to generate an X-ray projection image of the subject; a positioning-image creating unit that creates positioning images corresponding to a plurality of different projection conditions using volume data collected from the subject by a medical imaging apparatus; a projection-condition obtaining unit that selects 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 that creates and displays 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.