1. An image-capturing device that generates a wide dynamic range image through image synthesis, comprising: an image-capturing unit that obtains image data by executing photoelectric conversion on a light flux received at an image sensor; a dynamic range widening determination unit that determines that a dynamic range of an image is to be widened when a subject lightness value is equal to or lower than a lightness value upper limit which is greater than a predetermined lightness value provided that at least either the subject lightness value is equal to or greater than the predetermined lightness value or that an image-capturing frame rate is equal to or higher than a predetermined frame rate; an exposure control unit that controls exposure at the image sensor based upon determination results indicating whether or not to widen the dynamic range; an image synthesis unit that generates synthetic image data by combining image data in a plurality of frames captured over varying exposure times when a determination to widen the dynamic range is made; and a processing unit that displays or records the synthetic image data; wherein the exposure control unit lengthens both a long exposure time set for acquisition of long exposure image data and a short exposure time set for acquisition of short exposure image data as the subject lightness value decreases, and once the long exposure time set for acquisition of the long exposure image data matches a predetermined exposure time determined in correspondence to the image-capturing frame rate, the exposure control unit sustains the long exposure time set for acquisition of the long exposure image data at the predetermined exposure time determined in correspondence to the image-capturing frame rate even if the subject lightness value further decreases.