An X-ray diaphragm including plural sets of X-ray absorbing plates, each of the plural sets including four of X-ray absorbing plates, the four X-ray absorbing plates in each of the plural sets being arranged such that two of the four plates and the other two are positioned respectively in two directions perpendicular to each other to define a quadrangular aperture and that adjacent ones thereof respectively overlap each other partially at four corners of the aperture, the apertures in the plural sets of X-ray absorbing plates being coaxial and analogous apertures positioned and increasing in size in order in an X-ray irradiating direction, and a support device for supporting the plural sets of the plates such that the plates located on the same sides in the plural sets are supported respectively by the support device throughout the plural sets.