1. An image processing system comprising: an obtaining unit configured to obtain a plurality of tomographic images of an eye to be examined; an extraction unit configured to extract a retinal layer from the plurality of tomographic images; a generation unit configured to generate three-dimensional shape data of the retinal layer based on the retinal layer extracted by the extraction unit; a setting unit configured to set a flat surface that is movable in a depth direction of the retinal layer extracted by the extraction unit at a reference position of the three-dimensional shape data of the retinal layer; a measurement unit configured to measure at least one of an area and a volume of the retinal layer, based on the three-dimensional shape data of the retinal layer and the flat surface at the reference position, wherein the area is an area of the flat surface surrounded by the three-dimensional shape data of the retinal layer, and the volume is a volume of a region surrounded by the three-dimensional shape data of the retinal layer and the flat surface; and a display control unit configured to display at least one of the area and the volume on a display device.