1. An X-ray CT apparatus comprising: an X-ray source which generates an X-ray beam diverging along a body axis of a subject; an 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; a 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, and determining parts of the first projection data that contribute to image reconstruction; an 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 the determined parts of the first projection data that contribute to image reconstruction; and a 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.