1. An X-ray CT apparatus having an X-ray source which generates an X-ray beam diverging along a body axis of a subject, and X-ray detecting unit including detecting elements arranged in rows along the body axis of the subject and configured to detect X rays that have passed through the subject, the apparatus comprising: scan control unit for performing control, thereby acquiring first projection data while moving the X-ray source, relative to the subject, along the body axis of the subject, and acquiring second projection data while rotating the X-ray source around the subject; X-ray shielding unit for changing, while the first projection data is being acquired, an amount by which the X-ray beam generated by the X-ray source is shielded in accordance with a position where the first projection data is acquired; and reconstruction unit for synthesizing the first projection data partly shielded by the X-ray shielding unit, with the second projection data, thereby performing a reconstruction process.