Abstract: The radiation detector of the present invention comprises a joined scintillator block body which is composed of a plurality of photomultiplier tubes combined together and a plurality of scintillator blocks which are joined to entrance windows of the combined photomultiplier tubes so that the size of the entrance windows is in agreement with that of the light-outgoing plane of the scintillator blocks, the number of the scintillator blocks being greater than that of the photomultiplier tubes. The joined scintillator block body is designed such that the reflectances of the interfaces between the scintillator blocks increase as they are positioned away from the interfaces of the combined photomultiplier tubes.