1. A timing correction method for use in counting coincidences of radiation released from a positron-emitting radioactive drug given to a patient, the timing correction method comprising: a histogram correcting step, 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, for: (i) selecting a first detector as a reference detector and a second detectors as a count detector which count coincidences, to obtain a time lag histogram of the first detector and the second detector, and selecting a group of detectors including the second detector, (ii) selecting a detector different from the second detector from the group of detectors, (iii) correcting 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 selecting a detector, which has not been previously selected, from the group of detectors, and correcting 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, (v) repeating (iv) until all of the detectors in the group of detectors are subjected to (iv), and repeating (i)-(v) by selecting a new reference detector, a new counter detector and a new group of detectors until all of a plurality of detectors in a positron CT apparatus are subjected to the selection and correction, the new reference detector being selected from a previously selected group of detectors.