1. An image processing apparatus comprising: a tomographic image acquiring unit configured to acquire a plurality of polarization-sensitive tomographic images of an eye; an extracting unit configured to extract a depolarizing region from the plurality of polarization-sensitive tomographic images; a detecting unit configured to detect motion of the eye in the plurality of polarization-sensitive tomographic images, based on the extracted depolarizing region; and a positioning unit configured to position a plurality of tomographic luminance images corresponding to the plurality of polarization-sensitive tomographic images, based on the detected movement.