An X-ray detector of an X-ray CT apparatus has a collimator, a plurality of scintillators, a light reflector and a plurality of photodiodes. The collimator has a threshold plate with a thickness Wc to eliminate scattered radiation from an X-ray. The plurality of scintillators emit light based on the X-ray. The light reflector is provided in a gap between adjacent scintillators of the plurality of scintillators. The plurality of photodiodes convert the light of each of the plurality of scintillators into the electric signal. The thickness Wc of the threshold plate mounted on the X-ray incident side of the adjacent scintillators, and a thickness Ws of the gap has a relationship shown in a following expression: Wc≧Ws.
