According to one embodiment, a TOF-PET apparatus includes a plurality of detector rings arranged along a central axis thereof. Each of the detector rings comprises a plurality of scintillators and a plurality of photomultipliers. The scintillators are arranged on a substantial circumference around the central axis and generate scintillation in response to pair annihilation gamma-rays from a subject. The photomultipliers generate an electric signal in accordance with the generated scintillation. A length of each of the scintillators along a radial direction of the substantial circumference is set to a range in which a value of a total number of counts/time resolution of coincidence events of pair annihilation gamma-rays is more improved than when a reference scintillator whose probability of interaction with pair annihilation gamma-rays is adjusted to 80% is used under conditions of a constant total volume of the scintillators.