1. An image processing apparatus, comprising: an image acquiring unit configured to acquire a three-dimensional image of a fundus of an eye to be inspected based on a plurality of combined light beams obtained by combining a plurality of return light beams from the eye to be inspected that is irradiated with a plurality of measuring light beams, and a plurality of reference light beams respectively corresponding to the plurality of return light beams; a detection unit configured to detect a retina layer from each of a plurality of tomographic images obtained by photographing the fundus at a predetermined timing among a plurality of tomographic images constituting the three-dimensional image; an estimation unit configured to estimate a fundus shape based on the detected retina layer; and a correction unit configured to correct a positional deviation between the plurality of tomographic images constituting the three-dimensional image based on the estimated fundus shape.