According to one embodiment, the X-ray tube generates X-rays. The X-ray detector detects the X-rays transmitted through a subject. The data acquisition circuitry acquires count data concerning a count number of the detected X-rays for energy bands. The memory circuitry stores data of a response function that associates incident X-rays on the X-ray detector with a response characteristic of a system including the X-ray detector and the data acquisition circuitry. The processing circuitry calculates an X-ray absorption amount of each of a plurality of base substances based on the count data concerning the energy bands acquired by the data acquisition circuitry, an energy spectrum of the incident X-rays, and the response function.
