1. A scan type digital X-ray imaging apparatus comprising an X-ray generator for emitting X-ray slit beams, an X-ray detector for detecting X-ray transmitted through an object to be examined, and an object fixing means disposed between the X-ray generator and the X-ray detector, for holding the object in an appropriate position for X-ray photography, in which X-ray beam scanning is performed in a manner that the X-ray slit beam is emitted from said X-ray generator for said object while said X-ray generator and said X-ray detector are synchronously moved in the same direction with said object interposed therebetween, thereby producing an X-ray image of the object, wherein the scanning speed of said X-ray slit beam is controlled depending on the X-ray transmitted amount through said object detected by said X-ray detector during said X-ray beam scanning.