1. An X-ray diagnostic apparatus comprising: an image generating unit which generates first X-ray images before injection of a contrast medium and second X-ray images after injection of the contrast medium; a region detecting unit which detects a non-contrast region based on the first X-ray images and the second X-ray images; a selecting unit which selects one of the first X-ray images with respect to each of the second X-ray images on the basis of a correlation between the first X-ray images in the non-contrast region and the second X-ray images in the non-contrast region; and a subtraction image generating unit which generates subtraction images by performing subtraction between each of the second X-ray images and the selected one of the first X-ray images.