According to embodiment, an X-ray computed tomography apparatus includes an X-ray tube, an X-ray detector including a scintillator generating scintillation light upon incidence of X-ray photons and a photodetection element, a peak value detector detecting peak values corresponding to X-ray photons based on an output signal from the element, processing circuitry determining an attenuation characteristic of the light by each X-ray photon and an output decreased characteristic of the element, based on the values and time when each peak value was detected, correcting the detected values according to the characteristics, a counter counting the X-ray photons corresponding to the respective corrected peak values, wherein the processing circuitry reconstructs a medical image based on an output from the counter.
