1. An X-ray CT apparatus for taking an image with a first material of two materials within a subject to be examined being fractionated, by using a first set of X-rays at a first energy level and a second set of X-rays at a second energy level, said X-ray CT apparatus comprising: a data acquisition device configured to acquire a first set of data corresponding to the first set of X-rays and a second set of data corresponding to the second set of X-rays by scanning using the first set of X-rays and the second set of X-rays; a first image generating device configured to generate a first image based on the first set of X-rays and the second set of X-rays, the first image having pixel values made by weighted addition of first CT values corresponding to the first set of X-rays and second CT values corresponding to the second set of X-rays; a second image generating device configured to generate a second image based on the first set of data and the second set of data, wherein a second material of the two materials is fractionated in the second image; and a third image generating device configured to generate the image by weighted subtraction of the first image and the second image.