1. An image processing method of correcting a gain difference between pixels for an image signal obtained by an image sensing element having a plurality of pixels, comprising: sensing a first reference image without any object; sensing an object; performing gain correction for a sensed object image by using the first reference image and determining whether to perform gain correction again; when gain correction is to be performed again, sensing a second reference image without any object; and performing gain re-correction for the object image having undergone gain correction by using the first and second reference images.