A positron emission tomography device comprising: a group of PET gamma ray detectors adapted to receive pairs of pair-annihilation gamma rays from a subject to be imaged; and an imaging processor that receives a pair-annihilation detection signal supplied according to coincidence measurement with a pair of PET gamma ray detectors in the group of PET gamma ray detectors, wherein both of a first probe and a second probe are administered to the subject to be imaged, the first probe having a positron emitting nuclide that becomes an excited state of a daughter nuclide through beta decay and, following a positron emission by the beta decay, emits a unique gamma ray of a first energy during transition into a ground state of the daughter nuclide, and the second probe having a positron emitting nuclide that becomes an excited state of a daughter nuclide through beta decay and, following a positron emission by the beta decay, emits a unique gamma ray of a second energy during transition into a ground state of the daughter nuclide, the first energy and the second energy being different with each other, in that the pairs of pair-annihilation gamma rays are generated by positron-electron pair annihilations due to the first probe and the second probe, in that the PET device further comprises a unique gamma ray detector, which is separate from the group of PET gamma ray detectors, adapted to receive the unique gamma rays of the first energy and the second energy that are to be emitted by the first and the second probes respectively, in that the imaging processor executes reconstruction of distribution images for the pair-annihilation detection signal (S-EV) supplied based on coincidently detected pairs of pair-annihilation gamma rays by the group of PET gamma ray detectors and a signal supplied by the unique gamma ray detector differently according to whether an energy value of a unique gamma ray detected by the unique gamma ray detector within a predetermined time of the detection of each pair of the pair-annihilation gamma rays corresponds to the first energy or the second energy, and characterised in that the unique gamma ray detector is arranged in such a way that any straight line connecting each unique gamma ray detector and each PET gamma ray detector (10A, 10B) does not pass through field-of-view for the group of PET gamma ray detectors. a group of PET gamma ray detectors adapted to receive pairs of pair-annihilation gamma rays from a subject to be imaged; and an imaging processor that receives a pair-annihilation detection signal supplied according to coincidence measurement with a pair of PET gamma ray detectors in the group of PET gamma ray detectors, wherein both of a first probe and a second probe are administered to the subject to be imaged, the first probe having a positron emitting nuclide that becomes an excited state of a daughter nuclide through beta decay and, following a positron emission by the beta decay, emits a unique gamma ray of a first energy during transition into a ground state of the daughter nuclide, and the second probe having a positron emitting nuclide that becomes an excited state of a daughter nuclide through beta decay and, following a positron emission by the beta decay, emits a unique gamma ray of a second energy during transition into a ground state of the daughter nuclide, the first energy and the second energy being different with each other, in that the pairs of pair-annihilation gamma rays are generated by positron-electron pair annihilations due to the first probe and the second probe, in that the PET device further comprises a unique gamma ray detector, which is separate from the group of PET gamma ray detectors, adapted to receive the unique gamma rays of the first energy and the second energy that are to be emitted by the first and the second probes respectively, in that the imaging processor executes reconstruction of distribution images for the pair-annihilation detection signal (S-EV) supplied based on coincidently detected pairs of pair-annihilation gamma rays by the group of PET gamma ray detectors and a signal supplied by the unique gamma ray detector differently according to whether an energy value of a unique gamma ray detected by the unique gamma ray detector within a predetermined time of the detection of each pair of the pair-annihilation gamma rays corresponds to the first energy or the second energy, and characterised in that the unique gamma ray detector is arranged in such a way that any straight line connecting each unique gamma ray detector and each PET gamma ray detector (10A, 10B) does not pass through field-of-view for the group of PET gamma ray detectors.