Provided is a radiation detector that prevents a decline in detection efficiency as well as having excellent temporal characteristics. The radiation detector 1, which detects a radiation, includes a scintillator array 10 having a plurality of scintillator cells 11, a photodetector array 20 having a plurality of photodetectors 21, and a photodetector array 30 having a plurality of photodetectors 31. The plurality of photodetectors 21 and the plurality of photodetectors 31 are solid-state photodetectors that can transmit a gamma ray G1, G2. The scintillator cell 11 is separated into a plurality of light emitting regions 11a, 11b by a reflecting region 12, and the reflecting region 12 extends between the incident surface 10a side and the back surface 10b side along a surface SP that is inclined with respect to the incident surface 10a and the back surface 10b.