There is provided a calibration apparatus used for a photon counting type of radiation detector. In this apparatus, a radiation condition of a radiation is set such that particles of the radiation (X-rays) which are incident on a plurality of detection modules are piled up over each other at a probability which is equal to or less than a predetermined value. Under the setting of this radiation condition, detection sensitivities for the radiation are made uniform among the plurality of detection modules. Using this uniformed result, the detection sensitivities for the radiation are further made uniform every channel of each of the pixels formed by circuit groups including the plurality of detection modules, discrimination circuits and data calculation circuits and every discrimination circuit in each channel.
