PROBLEM TO BE SOLVED: To provide a radiation detector that enables a scintillator element array or a photodiode element array to improve accuracy of a relative position to a detection element module substrate without projecting from the detection element module substrate and that dispenses with highly accurate substrate dimensions.SOLUTION: There is provided a radiation detector comprising: a plurality of detection element modules each including a scintillator element array constituted by a plurality of scintillator elements each emitting light in response to the incidence of a radiation and reflectors isolating the adjacent scintillator elements a photodiode element array constituted by a plurality of photodiode elements detecting the light emitted from the scintillator elements and a detection element module substrate on which the scintillator element array and the photodiode element array are installed, a surface of the detection element module substrate on which surface the scintillator element array and the photodiode element array are installed being larger in substrate dimensions than an opposed surface.
