Radiation detector

A radiation detector for a PET-MR apparatus, has a fastening-strength and a high cooling efficiency relative to a photoelectric conversion element. A holding member 31 holds a side-periphery and a bottom of a light detector 7 by a connection element 31a, and is strongly fastened by a connection base 27 and a gimlet hole 31b and a screw 33. A light shielding film 35 is a thin-film and adheres and covers an outer-periphery and the side-periphery of the holding member 31. The radiation detector has a robust fastening-strength, so that a locational shift of the scintillator block 3 is prevented. As a result, the radiation detector can detect γ-ray data accurately.