1. An image-capturing device, comprising: an image-capturing circuit; and an image processing circuit, wherein the image-capturing circuit includes image-capturing element units arranged in a two-dimensional matrix form, and each of the image-capturing element units includes: a first image-capturing element including a first filter and a first photoelectric conversion element, and wherein the first image-capturing element is configured to receive light in a first wavelength band having a first peak wavelength; a second image-capturing element including a second filter and a second photoelectric conversion element, and wherein the second image-capturing element is configured to receive light in a second wavelength band having a second peak wavelength longer than the first peak wavelength of the first wavelength band; a third image-capturing element including a third filter and a third photoelectric conversion element, and wherein the third image-capturing element is configured to receive light in a third wavelength band having a third peak wavelength longer than the second peak wavelength of the second wavelength band; and a fourth image-capturing element including a fourth filter and a fourth photoelectric conversion element, and wherein the fourth image-capturing element is configured to receive light in the first wavelength band, the second wavelength band, and the third wavelength band, wherein a light transmission rate of the fourth filter is less than a light transmission rate of the first filter, a light transmission rate of the second filter, and a light transmission rate of the third filter, and the image processing circuit is configured to generate high sensitivity image data based on outputs from the first image-capturing element, the second image-capturing element, and the third image-capturing element, and generate low sensitivity image data based on an output from the fourth image-capturing element, wherein the image processing circuit is further configured to generate a combined image based on the high sensitivity image data corresponding to a low illumination image area in the low illumination image area obtained from the low sensitivity image data or the high sensitivity image data, and based on the low sensitivity image data corresponding to a high illumination image area in the high illumination image area obtained from the low sensitivity image data or the high sensitivity image data, wherein the image processing circuit includes a first gain adjustment unit configured to adjust the outputs from the first image-capturing element, the second image-capturing element, and the third image-capturing element, and a second gain adjustment unit configured to adjust the output from the fourth image-capturing element, and wherein the first gain adjustment unit and the second gain adjustment unit are implemented as a part of the image processing circuit.