Abstract: Provided is a calibration method for a detector including a plurality of detecting elements each of which generates an electrical pulse signal with a peak value corresponding to an energy value of incident X-ray photons and counts a photon count for each peak value, including: applying a predetermined tube voltage to an X-ray tube for irradiating the detector with X-rays; acquiring the photon count for each peak value from each of the detecting elements; estimating a maximum peak value H within a range in which X-ray photons are detected and at which the peak value is maximum; and calculating, for each of the detecting elements, a calibration value that associates the tube voltage with the maximum peak value H.