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; and an output unit configured to output an extraction result obtained by the extraction unit or information obtained based on the extraction result.