[en] In the simultaneous bi-plane system, one of the most important problems is the deterioration of radiographic image quality by the scattered X-rays from many direction. We met with good results by using the rare earth substance for the sake of deteriorating the side scattered X-rays. Then we compared the effects of elimination of the side scattered X-rays by using aluminum with that by using some rare earth substance and studied by X-ray spectrum theoretically. From the above-mentioned things, we got the best effect when the absorption edge of rare earth substance agreed with the region of energy of the side scattered X-rays. (author)