1. An X-ray computed tomography apparatus comprising: an X-ray tube configured to generate X-rays diverging in channel and slice directions; an X-ray detector which has a plurality of X-ray detection elements for detecting X-rays transmitted through an object to be examined, the plurality of X-ray detection elements being arrayed in the channel and slice directions; a rotating mechanism configured to support said X-ray tube and said X-ray detector so as to allow said X-ray tube and said X-ray detector to rotate around the object; a moving mechanism configured to move said X-ray tube and said X-ray detector relative to the object in the slice direction; a control unit configured to control said rotating mechanism and said moving mechanism to execute helical scan operation of acquiring data while making said X-ray tube and said X-ray detector rotate around the object and move relative to the object; an input device configured to set a substantially cylindrical reconstruction area; an image reconstructing unit configured to reconstruct image data within the set reconstruction area on the basis of an output from said X-ray detector; and a movement distance determining unit configured to determine a movement distance of said X-ray tube and X-ray detector relative to the object during generation of the X-rays on the basis of a radius and height of the set reconstruction area.