PROBLEM TO BE SOLVED: To accurately obtain noise intensity by a projection data division method using data after fan-para conversion.SOLUTION: In an image reconstruction device: a plurality of pieces of projection data acquired by irradiating radiation onto a subject are received, and the data is subjected to predetermined data conversion; the data after the conversion is divided into two or more sets, a reconstruction image is generated for each piece of data of each set, the generated reconstruction image of each set is differentiated, and a difference image is generated; an index is calculated, which indicates variations in pixel values for one or more predetermined regions of the difference image; and the value of the index is corrected by a correction value obtained beforehand, and the index value after the correction is made the noise intensity of the region.SELECTED DRAWING: Figure 7【課題】ファン-パラ変換後のデータを用いて投影データ分割法により精度よくノイズ強度を求める。【解決手段】被検体に放射線を照射して得た複数の投影データを受け取って、所定のデータ変換を行い、前記変換後のデータを、2以上の集合に分割し、各集合のデータごとに再構成画像を生成し、生成した前記各集合の前記再構成画像を差分し、差分画像を生成する。前記差分画像の所定の1以上の領域について画素値のばらつきを示す指標を算出し、前記指標の値を、予め求めておいた補正値によって補正し、補正後の前記指標値を前記領域のノイズ強度とする。【選択図】 図7