1. A CT scanner comprising: an acquisition unit configured to acquire a projected image by using a two-dimensional radiation sensor placed to face a radiation source through an object while relatively rotating the object in radiation emitted from the radiation source; a first reconstruction unit configured to obtain a plurality of tomographic images at one predetermined slice position of the object by performing, for each of a plurality of geometrical calibration parameter values, reconstruction of a tomographic image of the object using one of the plurality of geometrical calibration parameter values based on a projected image acquired by the acquisition unit; a selection unit configured to select one of the plurality of geometrical calibration parameter values based on the plurality of tomographic images obtained by the first reconstruction unit; and a second reconstruction unit configured to reconstruct a tomographic image at each slice position of the object by using a geometrical calibration parameter value selected by the selection unit based on a projected image at each slice position which is acquired by the acquisition unit.