With a view to providing a beam diaphragm having a large maximum value of aperture opening under a limited profile dimension, the beam diaphragm comprises a pair of control rings having coaxial apertures for the passage of X-rays therethrough and being opposed to each other axially through a spacing and coaxially rotatable independently of each other, a blade positioned between the pair of control rings, and position adjusting means which, in accordance with a relative rotation of the pair of control rings, causes the blade to move toward or away from a common axis of the apertures so as to describe a sectorial plane whose radius increases or decreases continuously.