Reconstruction operation device which can improve the image quality while reducing the computational amount when reconstructing an X-ray CT image by successive approximation, reconstruction calculation method, and to provide an X-ray CT apparatus, is divided into the air space 53 and other subject areas 52 and bed region 51, the initial image 50 is reconstructed by the analytical method of the FBP method, the reconstruction operation unit 45, converges at least each area of the divided updating the estimated image sequentially by setting the reconstruction conditions including the pixel size, performing approximation sequential processing each subset of other convergence reprojection data or estimation image sequential subset satisfies the convergence condition I be fixed until the condition is satisfied.逐次近似法にてX線CT画像を再構成する際の計算量を低減させつつ画質を向上させることが可能な再構成演算装置、再構成演算方法、及びX線CT装置を提供するために、再構成演算装置45は、FBP法等の解析的手法によって再構成された初期画像50を、寝台領域51と被検体領域52とその他の空気領域53とに分割し、分割した領域毎に少なくとも収束条件と画素サイズを含む再構成条件を設定して、それぞれ逐次近似処理を行い逐次推定画像を更新し、収束条件を満たした部分集合の逐次推定画像又は再投影データを、他の部分集合が収束条件を満たすまで固定とする。
