A nuclear medicine diagnosis apparatus comprising: a light signal generating unit configured to repeatedly generate light signals, wherein the light signal generating unit is joined to a plurality of photodetectors via a plurality of photoconductive paths, and the plurality of photoconductive paths have substantially the same length, each of the plurality of photodetectors comprises a scintillator configured to generate fluorescence upon receiving light, an electrical signal generating unit configured to generate an electrical signal upon receiving fluorescence generated by the scintillator, and wherein a light guide is placed between the scintillator and the electrical signal generating unit, and the light signal generating unit is configured to simultaneously apply the light signals to the respective light guides, or wherein no light guide is placed between the scintillator and the electrical signal generating unit, and the light signal generating unit is configured to simultaneously apply the light signals to the respective scintillators; a photodetection unit, wherein the photodetection unit includes the plurality of photodetectors provided around a long axis of a top, on which a subject is placed, and wherein the photodetection unit is configured to repeatedly detect the generated light signals, repeatedly generate first output signals corresponding to intensities of the detected light signals, repeatedly detect gamma rays emitted from the subject, and repeatedly generate second output signals corresponding to intensities of the detected gamma rays; a measurement unit configured to repeatedly measure light signal detection times of the light signals in the photodetection units and repeatedly measure gamma ray detection times of the gamma rays in the photodetection unit; a calculation unit configured to calculate a difference between a target gamma ray detection time and a target light signal detection time of the light signal detection times for each of the gamma ray detection times, the target light signal detection time being measured before the target gamma ray detection time by the same photodetector; a storage unit configured to store the calculated difference in association with a target second output signal of the second output signals; and an identifying unit configured to identify a pair of second output signals, of the second output signals, which originate from a pair of annihilation gamma rays by using the difference as a time stamp. a light signal generating unit configured to repeatedly generate light signals, wherein the light signal generating unit is joined to a plurality of photodetectors via a plurality of photoconductive paths, and the plurality of photoconductive paths have substantially the same length, each of the plurality of photodetectors comprises a scintillator configured to generate fluorescence upon receiving light, an electrical signal generating unit configured to generate an electrical signal upon receiving fluorescence generated by the scintillator, and wherein a light guide is placed between the scintillator and the electrical signal generating unit, and the light signal generating unit is configured to simultaneously apply the light signals to the respective light guides, or wherein no light guide is placed between the scintillator and the electrical signal generating unit, and the light signal generating unit is configured to simultaneously apply the light signals to the respective scintillators; a photodetection unit, wherein the photodetection unit includes the plurality of photodetectors provided around a long axis of a top, on which a subject is placed, and wherein the photodetection unit is configured to repeatedly detect the generated light signals, repeatedly generate first output signals corresponding to intensities of the detected light signals, repeatedly detect gamma rays emitted from the subject, and repeatedly generate second output signals corresponding to intensities of the detected gamma rays; a measurement unit configured to repeatedly measure light signal detection times of the light signals in the photodetection units and repeatedly measure gamma ray detection times of the gamma rays in the photodetection unit; a calculation unit configured to calculate a difference between a target gamma ray detection time and a target light signal detection time of the light signal detection times for each of the gamma ray detection times, the target light signal detection time being measured before the target gamma ray detection time by the same photodetector; a storage unit configured to store the calculated difference in association with a target second output signal of the second output signals; and an identifying unit configured to identify a pair of second output signals, of the second output signals, which originate from a pair of annihilation gamma rays by using the difference as a time stamp.