In order to provide a reconstruction computing device, a reconstruction computing method, and an X-ray CT apparatus capable of improving the image quality while reducing the amount of computation when reconstructing an X-ray CT image using an iterative method, a reconstruction computing device 45 divides an initial image 50 reconstructed using an analytical method such as an FBP method into a bed region 51, an object region 52, and the other air region 53, sets reconstruction conditions including at least convergence conditions and a pixel size for each of the divided regions, updates estimation images by performing an iterative process, and fixes estimation images or forward projection data of a subset satisfying the convergence conditions until other subsets satisfy the convergence conditions.