1. An apparatus for computed tomography image processing, comprising: an X-ray detection unit that detects an X-ray beam having passed through a subject and outputs an energy value thereof; a line integral calculation unit that calculates line integral values of attenuation coefficients representing attenuation of the energy value of the X-ray beam having passed through the subject and been detected, based on the energy value; an image processing unit that reconstructs a tomogram based on the line integral values; and an image output unit that outputs the tomogram, wherein the line integral calculation unit calculates, based on simultaneous equations obtained from a projection function of two different energy spectra and the energy value of the detected X-ray beam, line integral values of attenuation coefficients included in the projection function of the two different energy spectra and energy values satisfying the simultaneous equations.