An X-ray CT apparatus according to an embodiment includes an X-ray tube, a photon counting detector, and a processing circuitry. The X-ray tube is configured to generate X-rays. The photon counting detector includes a plurality of detecting elements each configured to output a signal in response to any of the X-rays becoming incident thereto after having passed through an examined subject. The processing circuitry is configured to determine, within a reconstruction region, a first region on which a spectrum reconstructing process is to be performed and a second region on which an energy integral reconstructing process is to be performed, on the basis of output values related to energy spectra based on the signals output by the detecting elements. The processing circuitry is configured to generate an image on the basis of the determined first region and the determined second region.
