An image processing apparatus is disclosed, which uses OCT to process a plurality of cross-sectional images obtained by moving an imaging core inside a catheter in an axial direction while rotating the imaging core. Cross-sectional image data is acquired in association with position information in the axial direction when each of the cross-sectional images. A first cross-sectional image in which a disappearance section enabling determination having a disappeared portion of an external elastic membrane included in a vascular tomographic image starts, and a second cross-sectional image in which the disappearance section ends are extracted from the plurality of cross-sectional images. The position information in the axial direction is acquired for the first cross-sectional image and the second cross-sectional image, and an ablation range influenced by ablation at a position associated with the disappearance section is calculated based on a difference in the acquired position information in the axial direction.