A center of gravity of a chest front image is obtained by a center of gravity calculation using pixel values in the chest front image, and a vicinity pixel position of the center of gravity is set as a center position P of a circular search region having a radius of a predetermined length. The shortest line segment of four line segments obtained by drawing perpendicular lines down from the center position P of an irradiation field to boundary lines of the upper, lower, left and right irradiation fields. The line segment, which is a radius of the circular search region, is rotationally displaced for each predetermined angle θ, and the profile of the average value for each predetermined angle θ is obtained. From the displacement angle θ of the line segment in which the average value becomes minimum, the orientation of the subject from the head to the abdomen reflected in the chest front image is determined. On the other hand, the displacement angle θ of the two line segments (diameter line segment) in which the sum of the average values of the two line segments (diameter line segments) opposed at 180° in the profile of the average value is obtained as an inclination angle of the chest front image.