1. An X-ray CT scanner, comprising: a gantry unit including an X-ray source configured to irradiate an object with X-rays and an X-ray detector configured to detect X-rays transmitted through the object, and configured to rotate the X-ray source and the X-ray detector while placing the X-ray source and the X-ray detector so as to make the X-ray source and the X-ray detector face each other; a reconstruction unit configured to generate reconstruction image data by using data acquired by the X-ray detector; an extraction unit configured to extract, when using a predetermined phantom as the object, pixel values of pixels existing on a locus surrounding a tomographic image of the phantom contained in reconstruction image data generated by the reconstruction unit based on data acquired by the X-ray detector, wherein the locus has a circular shape surrounding the tomographic image of the phantom; and an output unit configured to output an extraction result obtained by the extraction unit or information obtained based on the extraction result, wherein the output unit is configured to output a graph obtained by plotting pixel values, which are extracted by the extraction unit along a circumferential direction of the locus, as a function of angle.