To provide a fluorescent substance whose light-emitting output by X-rays is high and whose afterglow is very small, and to provide a radiation detector of high light-emitting output and low afterglow and an X-ray CT apparatus by which a tomogram of high resolution and a high quality is obtained.

In the radiation detector provided with a scintillator and a photodetector for detecting light emission by the scintillator, an oxide fluorescent substance which has a garnet structure with Ce as a light-emitting center ion and with Gd, Al, Ga and O as main elements and which contains at least one kind of element in groups IVa, Va and VIa of a long-period periodic table of ≥0.0001 mol% and ≤1.0 mol% to 1 mol% oxide fluorescent substance is used. Regarding the oxide fluorescent substance, the main peak of a light emitting spectrum is present near 535 nm, the light emitting output is more than 1.8 times as large as CdWO4, and the attenuation factor of the afterglow at 300 ms after cutting off stimulating light becomes ≤2.5×10-5. Thus, the radiation detector of high light-emitting output and low afterglow is obtained.