A diagnostic apparatus for nuclear medicine capable of performing an operation of counting gamma rays for a TCT simultaneously with an operation of counting gamma rays for an SPECT has a SPECT counter for counting, as the number of photons, gamma rays passed through a SPECT energy window centered at a photoelectric peak of the SPECT gamma rays and a TCT counter for counting, as the number of photons, gamma rays passed through a TCT energy window centered at a photoelectric peak of the TCT gamma rays. The number of photons of the gamma rays passed through the SPECT energy window contains the number of photons of K-X rays generated due to a photoelectric effect produced by the TCT gamma rays in collimators of the detector. A K-X ray processor estimates the number of photons of mixed K-X rays on the basis of the number of photons counted at the TCT counter. The K-X ray correction processor performs K-X ray correction on the photon number counted at the SPECT counter on the basis of the photon number of the estimated K-X rays. Since the number of photons passed through the TCT energy window and number of photons of K-X rays passed through the SPECT energy window have a very strong correlation from the standpoint of probability statistics, the accuracy with which the number of photons of the K-X rays passed through the SPECT energy window is estimated is very high. It is, therefore, possible to effectively reduce the K-X rays passed through the SPECT energy window.