A photon counting detector of an embodiment includes X-ray detection elements, a capacitor, and generating circuitry. The X-ray detection elements detect an X-ray and generate an electrical signal. The capacitor is provided for each of the X-ray detection element, and accumulates an electrical signal generated in each of the X-ray detection element. The generating circuitry has low sensitivity to radiation, and generates a digital signal by using an accumulation result of the electrical signal in the capacitors, and reference information that is stored in advance.