1. A computed tomography apparatus for performing a helical scan using a fan-shaped radiation beam, the apparatus comprising: first means for calculating 360.degree. virtual projection data at a first slice position by executing an estimation process of projection data in a range of 360.degree.+a double fan angle acquired before and after the first slice position, and reconstructing a tomographic image at the first slice position from the virtual projection data; second means for calculating a subtraction image between a tomographic image at a second slice position adjacent to the first slice position, and the tomographic image at the first slice position by reconstructing a difference between 360.degree. virtual projection data necessary for reconstructing the tomographic image at the second slice position, and the 360.degree. virtual projection data at the first slice position; and third means for calculating the tomographic image at the second slice position by synthesizing the subtraction image and the tomographic image at the first slice position.