To provide a tomographic image the picture quality level of which is uniform while suppressing the generation of artifacts in a CT system equipped with a rectangular two-dimensional X-ray detector.

The radius and the height of a desired cylindrical reconstruction region are input by a reconstruction region inputting means 310. A reconstruction-expanded-region calculating means 320 calculates the amount of expansion of the reconstruction region on the basis of the reconstruction region having been input and a region for which the projection data in a viewing direction are not complete. A mean for calculating the amount of expansion in a vertical direction of projection data 330 forms expanded projection data corresponding to the expanded amount of the reconstruction region by extrapolating or copying the projection data, and performs the reconstruction by using the expanded projection data and the projection data.