1. An image processing apparatus comprising: a superimposition unit configured to superimpose a plurality of captured images on each other so as to generate a superimposed image, wherein each captured image is obtained by an imaging unit and is of a fundus irradiated with a first amount of light and has a first quality, and wherein the superimposed image has a second quality which is higher than the first quality; a grayscale correction parameter setting unit configured to set a grayscale correction parameter for the superimposed image according to biological information of the fundus in the superimposed image generated by the superimposition unit; and an image correction unit configured to generate an image brighter than each said captured image on the basis of the grayscale correction parameter set by the grayscale correction parameter setting unit and the superimposed image generated by the superimposition unit.