To emit X-rays with equal beam size and uniform energy distribution regardless of energy by selecting X-rays having a necessary single number or a plurality of energies from X-rays having energies of wide range and transmitting them on the same optical axis to the position of a subject.

This X-ray CT instrument comprises a 2-crystal spectroscope consisting of a first crystal 11 and a second crystal 12, an uniforming filter 14 for uniforming the energy distribution of X-ray, and a detector 15. A necessary energy is selected by use of Bragg reflection in the first crystal 11, and the beam is extended by use of asymmetric reflection in the second crystal 12. The thickness of the uniforming filter 14 is set to the value t determined by the equation, T=(-1/μ)×log{I/f(x,y)} or an approximate value thereof when the horizontal direction of the plane orthogonal to the X-ray advancing direction is (x), the vertical direction of the plane orthogonal to the X-ray advancing direction is (y), the function showing the incident X-ray intensity distribution is (f), and the emitting X-ray intensity (constant) necessary in the subject position is I, and the absorption coefficient of the filter is μ.