1. An X-ray diagnostic apparatus comprising: an image generating unit which generates a plurality of X-ray images by repeatedly radiographing a subject before and after injection of a contrast medium; a region detecting unit which detects a non-contrast region from a plurality of mask images before injection of the contrast medium and a plurality of contrast images after injection of the contrast medium, which constitute said plurality of X-ray images; a mask selecting unit which separately selects one mask image from said plurality of mask images with respect to each of the contrast images on the basis of a correlation between the contrast image and the mask image upon localization to the non-contrast region; and a subtraction image generating unit which generates a subtraction image by performing subtraction between the contrast image and the selected mask image.