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.
