An X-ray computed tomography method for use with an X-ray computed tomography system having an X-ray source emitting X-rays to a test object, detectors installed opposite to each other in a manner flanking the test object, a control and image capturing device controlling the capture of projection data about the X-rays transmitted through the test object, a storage device storing a plurality of the projection data acquired by the detectors, and an image reconstruction computing device retrieving a plurality of the projection data from the storage device to perform image reconstruction computations in order to generate a tomographic image or a stereoscopic image, the X-ray computed tomography method comprising the steps of: performing a material count inputting process prompting a user to input the number of materials configuring the test object; performing an X-ray energy level setting process prompting the user to input the number of X-ray energy levels and energy levels, the number of X-ray energy levels being at least equal to the number of materials configuring the test object; performing a tomography process using a plurality of X-ray energy levels to carry out X-ray radiography a plurality of times based on the numbers set in the X-ray energy level setting process; performing a transmission length calculating process to calculate the transmission length of the material configuring a region of interest and the transmission length of the material configuring any region other than the region of interest; performing a region-of-interest projection data generating process of generating projection data about the region of interest by selecting one reference projection data set from among a plurality of projection data generated in the tomography process using a plurality of X-ray energy levels, wherein the selected reference projection data set is an X-ray attenuation rate at one energy level, and by subtracting from the reference projection data set the product of an attenuation coefficient and the transmission length of the material configuring any region other than the region of interest detected by detector elements of the detectors; and performing an image reconstruction computing process to generate a tomographic image or a stereoscopic image of the region of interest through image reconstruction based on the projection data about the region of interest generated in the projection data generating process of generating projection data about the region of interest. performing a material count inputting process prompting a user to input the number of materials configuring the test object; performing an X-ray energy level setting process prompting the user to input the number of X-ray energy levels and energy levels, the number of X-ray energy levels being at least equal to the number of materials configuring the test object; performing a tomography process using a plurality of X-ray energy levels to carry out X-ray radiography a plurality of times based on the numbers set in the X-ray energy level setting process; performing a transmission length calculating process to calculate the transmission length of the material configuring a region of interest and the transmission length of the material configuring any region other than the region of interest; performing a region-of-interest projection data generating process of generating projection data about the region of interest by selecting one reference projection data set from among a plurality of projection data generated in the tomography process using a plurality of X-ray energy levels, wherein the selected reference projection data set is an X-ray attenuation rate at one energy level, and by subtracting from the reference projection data set the product of an attenuation coefficient and the transmission length of the material configuring any region other than the region of interest detected by detector elements of the detectors; and performing an image reconstruction computing process to generate a tomographic image or a stereoscopic image of the region of interest through image reconstruction based on the projection data about the region of interest generated in the projection data generating process of generating projection data about the region of interest.