An X-ray CT apparatus according to an embodiment includes a photon counting detector, and processing circuitry. The photon counting detector includes a plurality of detecting elements each of which outputs signals corresponding to numbers of photons that are counted. The processing circuitry generates pieces of first projection data on a basis of the signals from the plurality of detecting elements. The processing circuitry generate second projection data by bundling together, in spatial units, pieces of first projection data from a predetermined number of detecting elements among the plurality of detecting elements. The processing circuitry reconstructs a material decomposition image on a basis of the second projection data.