1. An image processing apparatus comprising: one or more processors; and at least one memory storing instructions which, when executed by the one or more processors, cause the one or more processors to control: a calculation unit to calculate similarity among partial areas each of which is a partial area of each of a plurality of tomographic images of a subject's eye obtained based on measurement light of an optical coherence tomography (OCT) controlled to scan a single position for a plurality of times, each of the partial areas having a predetermined width in a direction orthogonal to an A-scan direction in each of the plurality of tomographic images; a selection unit to select a plurality of tomographic images corresponding to the partial areas the calculated similarity of which is higher than a predetermined value; and a generation unit to generate an addition average image by adding and averaging the selected plurality of tomographic images for enhancement of image quality of a tomographic image in an eye portion of the subject's eye.