Both an X-ray source and an X-ray detector are moved in a pitch which is smaller than a width of this detector. Since both the X-ray source and the X-ray detector are rotated by an angle of 360 within one move distance, a CT measurement is carried out. An X-ray detection amount of a slice width narrower than an X-ray effective detection width is calculated based upon the X-ray detection amount measured in the above-described manner.