1. An X-ray diagnostic apparatus comprising: an X-ray image generating unit which generates a plurality of X-ray images associated with a subject to be examined; a storage unit which stores data of a three-dimensional image associated with the subject; an image processing unit which generates data of a two-dimensional blood vessel image from the stored data of the three-dimensional image; a difference processing unit which generates a plurality of difference images based on the plurality of X-ray images; and a display unit which superimposes and displays each of the plurality of difference images and the two-dimensional blood vessel image.