An image processing apparatus which can always accurately extract, from a photographed image, an optimum feature quantity to be used for image processing is provided. In the apparatus, a limitation unit limits a predetermined area to extract the feature quantity in a lung area of an image (lung image obtained by radiographing). For example, as the predetermined area, the limitation unit limits an area obtained by dividing the lung area in a predetermined ratio, an area extending from 1/4 to 1/2 from a head side of a maximum-length vertical axis of the lung area, or an area within a predetermined-length range from the upside of the lung area. An extraction unit extracts a feature quantity (maximum pixel value) from the predetermined area.