A photon-counting X-ray CT apparatus in an embodiment includes an X-ray tube, an X-ray detector, correction circuitry, and processing circuitry. The X-ray tube is configured to irradiate a subject with X-rays. The X-ray detector is configured to include an X-ray detection element that detects photons of the X-rays that the X-ray tube emits. The correction circuitry is configured to correct a count value observed by the X-ray detection element in each of a plurality of energy bins, based on at least one of the probability of escape in the X-ray detection element and the probability of crosstalk in the X-ray detection element. The processing circuitry is configured to generate a reconstruction image, based on the count values corrected by the correction circuitry.
