Whether a phenomenon of photon incidence on detectors is a double event or a single event is determined (step S1). When it is a double event, emission data is collected (S2), and is put to an image reconstruction process (S3). When it is a single event, on the other hand, the data is collected as data for calibration (S4), and is put to a calibration process (S5). Since the data for calibration is collected during a clinical practice, a PET apparatus can be calibrated frequently without lowering the operating ratio of the apparatus.