PURPOSE: To provide a positron decay displaying apparatus which does not require image reproducing treatment to obtain satisfactory resolution degree and any wobbling to specify the decay point, and has sufficient sensitivity even in the case the amount of a radioactive element necessary for one time of marking an organ is slight.

CONSTITUTION: This positron decay displaying apparatus comprises at least a pair of scintillators 28 positioned on the opposite to each other, photomultiplier tube units 30 respectively connected with each scintillator, a means 34 to measure the common gravity center of the position of the scintillation of the scintillators, a means 36 to determine the flying time of gamma rays detected simultaneously by respectively paired scintillators, a means 38 to specify the positron decay, and an image displaying means 40.