1. An X-ray computed tomography (CT) apparatus comprising: processing circuitry configured to: acquire projection data that is based on a spectrum representing an amount of X-rays with respect to energy of a radiation having passed through a subject; select a plurality of materials; generate, from the projection data, first density images for each of the selected materials; generate a monochromatic image of specific energy from the first density images; reconstruct the projection data corresponding to the specific energy to generate a reconstructed image; compare the monochromatic image and the reconstructed image; and provide a notification indicating a result of the comparison.