The purpose of the present invention is to easily obtain height information of a subject disposed inside an elongated tubular member. The image processing apparatus (8) according to the present invention includes: an image conversion unit (11) into which two images are input, and which sets the central axis of the optical system in each input image and performs polar coordinate conversion on each image using the set central axis, the two images being obtained by inserting, inside a tubular member to be observed along a longitudinal direction thereof, an optical system having a field of view in at least a 90 degree direction relative to a central axis and imaging the inner peripheral surface of the member being observed at different locations along the longitudinal direction； a correspondence point detection unit (12) which detects correspondence points of the two converted images which have been subjected to the polar coordinate conversion； and a distance calculation unit (13) which calculates the distance of the inner peripheral surface of the member being observed from the central axis of the optical system on the basis of the distance between the imaging locations of the two images along the longitudinal direction and the correspondence points of the two converted images detected by the correspondence point detection unit (12).L'objet de la présente invention est d'obtenir facilement des informations de hauteur d'un sujet se trouvant à l'intérieur d'un élément tubulaire allongé. L'appareil (8) de traitement d'images selon la présente invention comprend : une unité (11) de conversion d'images dans laquelle sont entrées deux images, et qui définit l'axe central du système optique dans chaque image entrée et effectue une conversion en coordonnées polaires sur chaque image en utilisant l'axe central défini, les deux images étant obtenues en insérant, à l'intérieur d'un élément tubulaire à observer le long de sa direction longitudinale, un système optique ayant un cha
