The apparatus is equipped with an intersection identification unit configured to generate a pixel value profile which is a profile indicating relevance between a position of each pixel in a pixel array crossing a contour and a lung field of a subject and a corresponding pixel value and a moving average profile which is a profile indicating relevance between the position of each target pixel and a moving average of a corresponding pixel value and search an intersection of both the profiles located at a position where the pixel value profile surpasses the moving average profile from a direction from a rear end to a front end of the pixel array.
