1. An X-ray computed tomography (CT) apparatus comprising: a photon-counting detector configured to include a plurality of X-ray detection elements detecting X-ray photons applied from an X-ray tube; correction circuitry configured to correct detection signals detected by the photon-counting detector for the plurality of X-ray detection elements, based on a centroid of an X-ray spectrum detected by the photon-counting detector; and reconstruction circuitry configured to reconstruct a CT image based on the corrected detection signals.