A plurality of images are photographed (S11) under photographing conditions wherein a plurality of images in different light source statuses can be photographed under a light source whose luminance changes with time, such as a fluorescent lamp. Based on the picked up images, a change of luminance with time is detected (S12), a shadow area is estimated by using the change of the luminance with time (S13), and image processing, such as area division based on synthetic and optical characteristics of a linearized image is performed (S14).