PURPOSE: To realize a high time resolution by placing a reflecting surface of a single characteristic by a different area at every scintillator.

CONSTITUTION: Reflecting surfaces 2aW2c of a single characteristic are formed as shown by a figure B by a different area at every scintillator, on the surface except emitting surfaces of scintillators 1AW1C for executing a light emission of the same characteristic to the same excitation. As for the scintillator 1A, the reflecting surface 2a is provided on all the surfaces except the emitting surface, and as for the scintillator 1B, the reflecting surface 2b is provided partially on all the surfaces except the emitting surface. Also, with regard to the scintillator 1C, as well, the reflecting surface 2c is provided by a smaller area than the reflecting surface 2b, on all the surfaces except the reflecting surface. Each scintillator contacts closely through a shielding member 4, and a scintillation light is subjected to a difference influence by the reflecting surfaces 2aW2c and made incident on a photomultiplier tube 10. Subsequently, it is converted to a photoelectron by a photoelectric cathode 11, and thereafter, multiplied by diodes 12, 13, and outputted from a pin 15 through an anode 14. By this output, which scintillator has emitted a light beam is discriminated by a deciding circuit 100, and a radiation detecting system having a high time resolution is formed.