1. An X-ray imaging apparatus comprising: a reconstructing unit configured to reconstruct a plurality of volumes relating to an object, the plurality of volumes related to the object comprising: a first volume being generated based on an object area as a base and intensities of pixels in an X-ray image of the object as a first height; a second volume being generated based on the object area as the base and intensities of pixels in a first reference image of the object as a second height, the first reference image being generated based on the object being of only adipose tissue; and a third volume being generated based on the object area as the base and intensities of pixels in a second reference image of the object as a third height, the second reference image being 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 first, second and third volumes, wherein the density calculator is further configured to calculate the density based on a difference between the first volume of the object reconstructed from the X-ray image and the second volume of the object reconstructed from the first reference image.