An X-ray CT apparatus according to an embodiment includes acquiring circuitry and processing circuitry. The acquiring circuitry is configured to count photons derived from X-rays that have passed through a subject and to acquire a result obtained by discriminating energy levels of the counted photons as a counting result. The processing circuitry is configured to notify the acquiring circuitry of an energy dividing set that is set in accordance with an X-ray absorption characteristic of a substance designated by an operator, to receive the counting result acquired by the acquiring circuitry by allocating a counted value to each of a plurality of energy discrimination regions that are set in the energy dividing set, and to reconstruct image data by using the received counting result.
