A positron CT apparatus having a plurality of detectors for detecting radiation released from a positron-emitting radioactive drug given to a patient, comprising: a computing device, in connection with a time lag histogram showing a count value distribution of time lag variations for each pair of detectors which count coincidences of the radiation, adapted to: (i) select a first detector as a reference detector and a second detector as a counter detector, which count coincidences to obtain a time lag histogram of the first detector and the second detector, and select a group of detectors including the second detector, (ii) select a detector different from the second detector from the group of detectors, (iii) correct a time lag histogram of the first detector and the selected different detector using the time lag histogram of the first detector and the second detector, (iv) newly select a detector, which has not been previously selected, from the group of detectors, and correct a time lag histogram of the first detector and the newly selected detector using the time lag histogram of the first detector and the second detector, and (v) repeat (iv) until all of the detectors in the group of detectors are subjected to (iv); and a coincidence counting circuit for counting coincidences of the radiation based on the time lag histogram for each pair of detectors repeatedly corrected by the computing device. a computing device, in connection with a time lag histogram showing a count value distribution of time lag variations for each pair of detectors which count coincidences of the radiation, adapted to: (i) select a first detector as a reference detector and a second detector as a counter detector, which count coincidences to obtain a time lag histogram of the first detector and the second detector, and select a group of detectors including the second detector, (ii) select a detector different from the second detector from the group of detectors, (iii) correct a time lag histogram of the first detector and the selected different detector using the time lag histogram of the first detector and the second detector, (iv) newly select a detector, which has not been previously selected, from the group of detectors, and correct a time lag histogram of the first detector and the newly selected detector using the time lag histogram of the first detector and the second detector, and (v) repeat (iv) until all of the detectors in the group of detectors are subjected to (iv); and (i) select a first detector as a reference detector and a second detector as a counter detector, which count coincidences to obtain a time lag histogram of the first detector and the second detector, and select a group of detectors including the second detector, (ii) select a detector different from the second detector from the group of detectors, (iii) correct a time lag histogram of the first detector and the selected different detector using the time lag histogram of the first detector and the second detector, (iv) newly select a detector, which has not been previously selected, from the group of detectors, and correct a time lag histogram of the first detector and the newly selected detector using the time lag histogram of the first detector and the second detector, and (v) repeat (iv) until all of the detectors in the group of detectors are subjected to (iv); and a coincidence counting circuit for counting coincidences of the radiation based on the time lag histogram for each pair of detectors repeatedly corrected by the computing device.