1. An image processing apparatus comprising: an acquisition unit configured to acquire projected images by using a two- dimensional radiation sensor placed to face a radiation source through an object while the two- dimensional radiation sensor and the radiation source are integrally rotating relative to the object and around the object; a reconstruction unit configured to reconstruct a plurality of tomographic images of the object from the acquired projected images using a plurality of calibration parameter values; a selection unit configured to select at least one of the plurality of calibration parameter values; and an output unit configured to output a tomographic image based on the selected at least one of the plurality of calibration parameter values, wherein the plurality of calibration parameter values include parameter values of a tilt of a rotation axis of the object relative to the radiation source and the two-dimensional radiation sensor.