1. A radiation tomography apparatus comprising: a first gantry which has an opening to receive a subject therethrough; a top board on which the subject is placed, the top board being attached to a device associated with the first gantry; a top board mover configured to move the top board in a direction in which the opening extends with respect to the first gantry; a second gantry which is attachable and removable from the device associated with the first gantry, the second gantry having an opening to receive the top board on which the subject is placed; a corrector configured to make a correction for positioning a subject image captured by the device associated with the first gantry with respect to a subject image captured by the device associated with the second gantry; and wherein the corrector makes the correction for positioning the subject images based on a calibration image captured to shoot, in a single field of view, a marker attached to the first gantry and a marker attached to the second gantry.