According to on embodiment, a photon counting CT apparatus includes a hybrid detector and processing circuitry. The hybrid detector includes CT detectors and PCCT detectors. The CT detectors output integral signals concerning the detected X-rays. The PCCT detectors output a count signal for each of a plurality of energy bands concerning the detected X-rays. The processing circuitry estimate a count signal concerning an estimation target CT detector of the CT detectors based on an integral signal output from the estimation target CT detector and a count signal output from the PCCT detector. And the processing circuitry reconstruct an image based on the estimated count signal and the count signal.