According to one embodiment, an X-ray CT scanner includes a gantry unit, a reconstruction unit, an extraction unit, and an output unit. The gantry unit includes an X-ray source and an X-ray detector and is configured to rotate the source and the detector. The reconstruction unit generates reconstruction image data by using data acquired by the detector. The extraction unit extracts, 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 detector. The output unit outputs an extraction result obtained by the extraction unit or information obtained based on the extraction result.