1. An X-ray CT device, including an X-ray generation unit for generating an X-ray, an X-ray detection unit for detecting the X-ray after transmitted through a subject, a projection data measurement unit for generating measurement projection data from a detection signal of the X-ray detection unit which has been measured by rotating the X-ray generation unit and the X-ray detection unit, and an image generation unit for generating a CT image from the measurement projection data, the image generation unit, comprising: a background image creation unit for creating a CT image of a background region from the measurement projection data; a background projection data calculation unit for calculating background projection data of the CT image of the background region on a path connecting the X-ray generation unit and the X-ray detection unit; a local measurement projection data calculation unit for calculating local measurement projection data of a local region using the measurement projection data and the background projection data; a local image calculation unit for calculating the CT image of the local region from the local measurement projection data; a local projection data calculation unit for calculating local calculation projection data of the CT image of the local region on the path connecting the X-ray generation unit and the X-ray detection unit; a local image correction unit for iteratively correcting the CT image of the local region on the basis of the local calculation projection data obtained by the local projection image calculation unit and the local measurement projection image, and a cause elimination unit for eliminating a cause to deteriorate calculation accuracy of the CT image of the background region or the background projection data.