1. A compound eye imaging apparatus which calculates a disparity occurring in a plurality of imaging optical systems that take an image of a same object, said apparatus comprising: a standard imaging optical system which generates a picture including a standard image by taking an image of the object; reference imaging optical systems of an even number equal to or larger than two, each of which has an optical center and generates a picture including a reference image by taking an image of the object, the optical centers being arranged substantially point-symmetrically with respect to an optical center of said standard imaging optical system; a correlation value calculation unit configured to calculate a correlation value indicating a degree of similarity between the standard image and the reference image, for each shifting amount by which a search position of the reference image in the picture generated by said reference imaging optical systems is shifted in a direction parallel to a baseline so that an image position of the reference image which is similar to the standard image is searched for, for each of said reference imaging optical systems of an even number equal to or larger than two, the baseline being a straight line connecting the optical center of said standard imaging optical system and the optical center of said reference imaging optical systems; a correlation value addition unit configured to calculate an integrated correlation value by adding the correlation value to each corresponding shifting amount, the correlation value being calculated for each of said reference imaging optical systems of an even number equal to or larger than two; and a disparity calculation unit configured to calculate, at a sub-pixel level, a disparity that is a shifting amount which results in a largest degree of similarity between the standard image and the reference image.