According to one embodiment, an X-ray computed tomography apparatus includes an X-ray tube, an X-ray detector, a projection data generation circuitry, a setting circuitry, and a scan start timing determination circuitry. The setting circuitry configured to set a region of interest on a slice image generated by a first scan for the object. The scan start timing determination circuitry configured to determine, based on a plurality of projection data values of interest corresponding to the region of interest out of the projection data values generated by a second scan at a dose lower than that in the first scan, a timing of terminating the second scan and starting a third scan at a dose higher than that in the second scan.
X ray computed tomography apparatus and scan start timing determination method