In a state where a subject 5 is not present, a pre-processing unit 7 changes the focal spot position that becomes the position of an X-ray tube 2, acquires the X-ray incidence rate of each detection module at each focal spot position, approximates the X-ray incidence rate of each detection module by the X-ray incidence rate of a module in which a reference detector is present, and stores the coefficient of an approximate polynomial in a storage unit 8. When a scanning of the subject 5 is performed, the X-ray incidence rate of each detection module is calculated using the X-ray incidence rate of the module in which the reference detector is present and the stored coefficient, and sensitivity correction data relating to each focal spot position is obtained using the calculated X-ray incidence rate when the scanning of the subject is performed.