A .gamma.-ray signal processing section 60' determines a detection time of a .gamma. ray based on a .gamma.-ray detection signal outputted from a semiconductor radiation detector for detecting the .gamma. ray, and determines the energy of the .gamma. ray. Then, a time correction circuit 70 obtains, based on the energy of the .gamma. ray, a detection value of the detection time that corresponds to the energy of the .gamma. ray from a time correction table indicating the relationship between the energy of the .gamma. ray and the correction value of the detection time of the .gamma. ray, and corrects the detection time according to the obtained correction value of the detection time. Coincidence counting is performed on the .gamma. ray in a coincidence counting circuit 80 based on the corrected detection time.