1. An X-ray imaging apparatus comprising: an X-ray image taking unit that takes an X-ray image by irradiating an X-ray to a subject and detecting the X-ray passed through the subject; a three-dimensional blood-vessel information creating unit that creates information concerning positions of a three-dimensional blood-vessel core line and a blood-vessel lesion-site inside a blood vessel to be imaged, based on three-dimensional volume data obtained from an image taken by a medical diagnostic imaging apparatus; a blood-vessel lesion-site-depth information image creating unit that creates a blood-vessel lesion-site-depth information image in which a display pattern of a blood-vessel lesion-site is changed in accordance with whether the blood-vessel lesion-site is present in front of or in the back of a three-dimensional blood-vessel core line, based on the information concerning positions of the three-dimensional blood-vessel core line and the blood-vessel lesion-site created by the three-dimensional blood-vessel-information creating unit; and an X-ray image display unit that displays the blood-vessel lesion-site-depth information image created by the blood-vessel lesion-site-depth information image creating unit in a superimposed manner over the X-ray image.