1. An imaging device comprising: an image sensor to convert an optical image of a subject into an electric signal; an image processor to generate an image according to the electric signal; an image synthesizer to synthesize, pixel-by-pixel, images generated by the image processor in a same composition under a same condition; a digital clamp processor to perform a digital clamp to remove an offset component from a synthesized image obtained by the image synthesizer; and a digital clamp controller to adjust a digital clamp amount of the digital clamp processor in correspondence with a number of images synthesized by the image synthesizer, wherein the image synthesizer includes a synthesizer to compare, pixel-by-pixel, on the basis of a brightness value, corresponding coordinates of the images generated in the same composition under the same condition and synthesize the images while replacing a pixel with at least either a pixel having a higher brightness value or a pixel having a lower brightness value.