1. An image processing device comprising: an acquisition unit configured to acquire a tomographic image of a retina of an eye to be examined; a generating unit configured to generate a new tomographic image, using the acquired tomographic image and a reverse image in which at least one portion of the acquired tomographic image is reversed, so that the reverse image is located on a side on which a retinal layer in the acquired tomographic image is in contact with an end of the acquired tomographic image; and a detection unit configured to detect at least one retinal layer in the acquired tomographic image, and to detect, based on continuity between the detected at least one retinal layer in the acquired tomographic image and at least one retinal layer in the reverse image, the at least one retinal layer in the new generated tomographic image.