1. A photon-counting X-ray CT apparatus comprising: an X-ray tube configured to irradiate a subject with X-rays; an X-ray detector including an X-ray detection element configured to detect photons of the X-rays that the X-ray tube emits; correction circuitry 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 probability of escape in the X-ray detection element and probability of crosstalk in the X-ray detection element; and processing circuitry configured to generate a reconstruction image, based on the count value corrected by the correction circuitry.