1. An image processing device that synthetically generates an image of a subject on a given focal plane based upon output data of a plurality of photoelectric conversion elements corresponding to each of a plurality of micro-lenses, the image processing device comprising: a drive control unit; and an image synthesis unit provided in the drive control unit that (1) synthetically generates a first pixel of the image corresponding to a first part in the given focal plane for the subject set based upon output data of first photoelectric conversion elements among the plurality of photoelectric conversion elements corresponding to a micro-lens among the plurality of micro-lenses, light from the first part entering the first photoelectric conversion elements, (2) synthetically generates a second pixel of the image corresponding to a second part in the given focal plane for the subject set based upon output data of second photoelectric conversion elements among the plurality of photoelectric conversion elements corresponding to the micro-lens among the plurality of micro-lenses, light from the second part entering the second photoelectric conversion elements, and (3) synthetically generates the image which includes more pixels than the plurality of micro-lenses, on the given focal plane.