The present invention provides a gadolinium oxysulfide sintered body having a high light output. The problem is resolved by a gadolinium oxysulfide sintered body in which the ratio of the light transmittance T410 of 410 nm to the light transmittance T512 of 512 nm (T410/T512) is from 0.31 to 0.61, or a gadolinium oxysulfide sintered body in which the ratio of the diffraction peak intensity Iy of a phase different from gadolinium oxysulfide appearing at 2θ=from 20 to 29° to the diffraction peak intensity (Ix) of (102) or (011) of gadolinium oxysulfide appearing at 2θ=30°±1° (Iy/Ix) is 0.1 or less in an XRD diffraction pattern and which contains one or more activators selected from the group consisting of praseodymium, terbium, and cerium.