According to one embodiment, a photon counting imaging apparatus includes an X-ray tube, an X-ray detector, and data acquisition circuitry. The X-ray detector includes a detector pixel including photoelectric conversion cells each configured to individually generate an electrical signal of a predetermined pulse height, and output circuitry configured to generate an energy signal having a pulse height corresponding to energy of the X-rays based on the electrical signals from the photoelectric conversion cells. The data acquisition circuitry corrects the energy signal based on a relationship between an amplification factor and an applied voltage to the detector pixel during a period until the voltage applied to the detector pixel recovers from a breakdown voltage to a reverse bias voltage.