In order to provide an X-ray CT device which has minimal decrease in image quality even though a tube current value of an X-ray tube is suppressed, this X-ray CT device comprises an X-ray tube, and is characterised by comprising: an X-ray source which irradiates a subject with X-rays； an X-ray detection apparatus which detects transmitted X-rays emitted from the X-ray source and transmitted through the subject； a rotating mechanism to which the X-ray source and the X-ray detection apparatus are mounted, and which rotates around the subject； a system control device which calculates a tube current value of the X-ray tube on the basis of a successive approximation processing condition selected from among a plurality of successive approximation processing conditions, and an inputted imaging condition and/or a reconstruction condition, and furthermore which carries out imaging using the calculated tube current of the X-ray tube； and an image reconstruction device which irradiates the subject from an X-ray source on the basis of the calculated tube current value of the X-ray tube, and which reconstructs a tomographic image of the subject from the amount of transmitted X-rays transmitted through the subject and detected by the X-ray detection apparatus, said reconstruction being on the basis of the selected successive approximation processing condition and the reconstruction condition.Pour qu'un dispositif CT à rayons X subisse une diminution minimale de qualité d'image même si une valeur de courant de tube d'un tube à rayons X est supprimée, le dispositif CT à rayons X selon l'invention comprend un tube à rayons X, et est caractérisé en ce qu'il comprend : une source de rayons X qui irradie un sujet avec des rayons X； un appareil de détection de rayons X qui détecte les rayons X transmis émis par la source de rayons X et transmis à travers le sujet； un mécanisme rotatif sur lequel la source de rayons X et l'appareil de détection de rayons X sont montés, et qui tourne auto
