1. A digital subtraction-imaging apparatus comprising: means for providing X-ray images through an object exposed to an X-ray source; means for injecting said object with an X-ray contrast medium; first memory means for temporarily storing a plurality of images, as mask images, which have been acquired during at least one cardiac cycle before said X-ray contrast medium reaches a region of interest (ROI) of the object under medical examination; second memory means for temporarily storing a plurality of images, as contrast images, which have been acquired during a plurality of cardiac cycles, succeeding said one cardiac cycle, after the X-ray contrast medium has reached the ROI of the object; means for performing digital subtraction in synchronism with a cardiac phase of said object, between said successive mask images, read out from the first memory means, and said corresponding contrast images, read out from the second memory means; and means for controlling said digital subtraction means in such a manner that when the number of contrast images acquired within an arbitrary single cardiac cycle defined by two successive cardiac phases is smaller than that of the mask images stored in the first memory means and acquired within said single cardiac cycle, said subtraction means performs digital subtraction between all of said contrast images and the corresponding mask images, withholding a predetermined number of said mask images which do not correspond to said contrast images, with respect to the cardiac cycles, whereas, when the number of contrast images acquired within said single cardiac cycle is greater than that of the mask images acquired within said single cardiac cycle; said digital subtraction is carried out by utilizing all of said contrast images and by repeatedly utilizing at least one of said mask images, in addition to all of the corresponding mask images, with respect to the cardiac cycles.