In an X-ray computed-tomography apparatus of an embodiment, a reconstruction processing unit reconstructs image data based on projection data that is generated from output of an X-ray detector. A specifying unit specifies a scan position or a scan range for main scanning. A changing unit identifies characteristic points based on a structure inside the subject from image data, and changes the scan position or the scan range specified by the specifying unit based on a result of checking data relating to the characteristic points and data relating to corresponding anatomical characteristic points in a virtual subject against each other.