A CT scanner which can reconstruct high-quality tomographic images using only the projected images obtained by scanning an object without requiring any special three-dimensional phantom is provided. A two-dimensional radiation sensor placed to face a radiation source via the object acquires projected images while relatively rotating the object in the radiation emitted from the radiation source. Tomographic images are obtained at one predetermined slice position of the object by performing, for each of geometrical calibration parameter values, reconstruction of a tomographic image of the object using one of the calibration parameter values based on acquired projected images. One of the calibration parameter values is selected based on the obtained tomographic images. A tomographic image is reconstructed at each slice position by using a selected calibration parameter value based on a projected image at each acquired slice position of the object.
