1. An X-ray imaging apparatus comprising: a reconstructing unit configured to reconstruct volumes relating to an object from an X-ray image of the object, a first reference image of the object generated based on the object being of only adipose tissue, and a second reference image of the object generated based on the object being of only fibroglandular tissue; and a density calculator configured to calculate a density of the object that is a ratio of fibroglandular tissue of the object with respect to entire tissue of the object using the reconstructed volumes of the object.