According to one embodiment, a photon counting CT apparatus includes an X-ray source, a photon counting CT detector, and a calibration unit. The X-ray source includes a cathode configured to generate electrons and an anode including a plurality of targets configured to generate a plurality of characteristic X-rays having different energies. The photon counting CT detector detects X-ray photons generated by the X-ray source. The calibration unit calibrates the gain of the photon counting CT detector based on the correspondence relationship between the photon energies of the plurality of characteristic X-rays and outputs from the photon counting CT detector.
