A PET apparatus and a timing correction method of this invention select two target gamma-ray detectors which count coincidences, select a reference detector which is one detector out of the two selected gamma-ray detectors, select a gamma-ray detector different from the other, opposite detector, and when repeating the selection, make a time lag histogram concerning two gamma-ray detectors selected in the past a reference, and correct a time lag histogram concerning gamma-ray detectors selected this time based on the reference. By repeating an operation to make the corrected time lag histogram concerning the two gamma-ray detectors a new reference, an optimal time lag histogram can be obtained without repeating many measurements and computations.
