A nuclear medicine diagnostic apparatus according to an embodiment includes a scintillator configured to be formed of a single crystal and convert a gamma ray into light； a plurality of photodetectors configured to be arranged on different faces or tangents of the scintillator and each of which is configured to output an electric signal in response to incidence of the light resulting from the converting by the scintillator； storage circuitry configured to store, in advance, correspondence information in which each position in the scintillator is associated with a first intensity distribution indicating intensities of the electric signals that are output by the respective photodetectors； and specifying circuitry configured to specify a conversion position in which the gamma ray that is emitted from the subject is converted into the light in the scintillator by using the correspondence information and a second intensity distribution indicating the intensities of the electric signals.
