The invention includes an insertion portion, one end of which is inserted inside a lumen of a subject an image-acquisition unit for acquiring an image of an interior wall in the lumen as a substantially circular or substantially ring-shaped interior wall image a transformation processing unit for transforming, in the interior wall image, at least substantially ring-shaped images concerning the interior wall, whose distance from the one end is uniform, to substantially straight-line-shaped or substantially band-shaped images a distance-measuring unit for measuring a distance moved by the insertion portion relative to the subject and an image synthesizing unit for joining the transformed images on the basis of the distance moved measured by the distance-measuring unit.
