An X-ray computed tomography (CT) apparatus according to an embodiment includes an X-ray generator, an X-ray detector and processing circuitry. The X-ray generator irradiates X-rays to a subject. The X-ray detector detects the X-rays that have passed through the subject. The processing circuitry calculates an estimated spectrum based on an irradiation spectrum, an estimated length and information indicating a distortion of a spectrum occurring in a path of the X-rays passing through the subject, the estimated length representing an estimated value of an X-ray transmission length of a material of decomposition target. The processing circuitry determines an X-ray transmission length of the material of decomposition target based on the estimated spectrum and a detected spectrum that is a spectrum after the X-rays have passed through the subject and that is detected by the X-ray detector.
