1. A method of reconstructing a tomography image of an object from a plurality of projection images generated by scanning the object in a plurality of projection directions, the method comprising: determining whether a truncated portion of the object is present in each of the plurality of projection images; reconstructing a first reconstruction image by using at least one projection image, in which a truncated portion of the object is absent, based on a result of the determining; calculating projection data for the first reconstruction image, by using the first reconstruction image; and reconstructing a tomography image of the object by using the calculated projection data.