1. An image processing apparatus comprising: a superimposition unit that superimposes a plurality of captured images on each other obtained by an imaging unit which images a fundus irradiated with a small amount of light, so as to generate a superimposed image; a grayscale correction parameter setting unit that sets 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 that generates an image brighter than the 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.