The center of gravity is calculated by the center of gravity calculation using the pixel value in the front chest image, and the position of a pixel in the vicinity thereof is set as the center position P of a circular search area having a radius of a predetermined length. The radius of the shortest line segment among the four line segments perpendicular to the boundary lines of the irradiation field in the vertical and horizontal directions from the center position P is defined as the radius. A line segment, which is the radius of the circular search area, is rotationally displaced at each predetermined angle θ, and a profile of an average value at each predetermined angle θ is obtained. The direction of the subject from the head to the abdomen in the chest front image is determined from the displacement angle θ of the line segment at which the average value becomes the minimum value. On the other hand, in the profile of the average value, the displacement angle θ of the two line segments (diameter line segments) at which the sum of the average values of the two line segments (diameter line segments) opposed by 180 ° is the minimum value is determined by the inclination of the chest front image. Obtain as an angle.胸部正面画像における画素値を用いた重心演算によって重心を求め、その近傍画素位置を、所定長さの半径を有した円形探索領域の中心位置Pとして設定する。中心位置Pから上下左右の照射野の境界線に垂線を下した４つの線分のうち最も短いものを半径とする。円形探索領域の半径である線分を所定角度θ毎に回転変位させて、所定角度θ毎の平均値のプロファイルを求める。平均値が最小値となる線分の変位角度θから、胸部正面画像に写り込む頭部から腹部に向かう被検体の向きを決定する。一方、平均値のプロファイルにおいて180°対向した２つの線分（直径線分）での平均値の和が最小値となる２つの線分（直径線分）の変位角度θを胸部正面画像の傾き角度として求める。
