In order to increase the recognition rate of a shadow area contained in the image of an aerial photograph, a central control portion performs the processes of: comparing a shadow-produced angle with the angles of segments constituting the polygons generated by a polygon-generating portion to judge whether or not there are the segments coinciding with the shadow-produced angle, of the segments constituting the polygon; judging whether or not the ratio of the length of the segment constituting the shadow to the length of the segment constituting the feature coincides with a shadow-extended ratio; judging whether or not the color tone of the area surrounded by the segments constituting the shadow is darker than that of all the adjacent polygons; and identifying the area surrounded by the segments constituting the shadow as “shadow area.” The polygon of the identified shadow area is generated.