A center of gravity is obtained by means of a center of gravity calculation using pixel values in a chest front image, and a pixel position in the vicinity thereof is set as a central position P of a circular search region having a radius of a prescribed length. The shortest of four line segments obtained by dropping perpendicular lines to boundary lines of an irradiation field above and below and to the left and right of the central position P is set as the radius. The line segment that is the radius of the circular search region is rotationally displaced through a prescribed angle θ, and an average value profile at each prescribed angle θ is obtained. The orientation of a subject, directed from a head portion to the chest portion appearing in the chest front image, is determined from the displacement angle θ of the line segment for which the average value is smallest. Meanwhile, the sum of the average values of two line segments (a diametric line segment) that oppose one another at 180° in the average value profile is obtained for each displacement angle θ, and the displacement angle θ of the two line segments (the diametric line segment) for which said sum is smallest is obtained as an angle of inclination of the chest front image.La présente invention concerne un centre de gravité obtenu à l'aide d'un calcul de centre de gravité en utilisant les valeurs de pixel d'une image avant de poitrine, et d'une position de pixel à proximité de cette dernière est paramétrée comme position centrale (P) d'une région de recherche circulaire présentant un rayon d'une longueur prescrite. Le plus court de quatre segments linéaires obtenus en faisant tomber des lignes perpendiculaires vers les lignes frontières d'un champ d'irradiation au-dessus et en-dessous et à gauche et à droite de la position centrale (P) est défini comme rayon. Le segment linéaire qui est le rayon de la région de recherche circulaire est déplacé de manière à pouvoir tourner selon un angle prescrit (θ), et u
