In processing an image signal output from an image sensing unit which includes a plurality of image sensing regions constructing one frame and a plurality of output terminals in one-to-one correspondence with the plurality of image sensing regions, and which outputs, from each of the plurality of output terminals, an image signal pertaining to image sensing in a corresponding image sensing region and a dummy image signal concerning the corresponding image sensing region, offset differences between a plurality of image signals output from the plurality of output terminals are reduced on the basis of a plurality of dummy image signals output from the plurality of output terminals. The plurality of dummy image signals are independent of electric charge obtained from photoelectric converters of the image sensing unit.