1. A water droplet detection device comprising: an image capturing unit configured to capture an area an image of a predetermined area, the image capturing unit having a photographic optical system; and a water droplet detection unit programmed to set an arbitrary attention point in the image obtained by the image capturing unit, a plurality of first reference points inside an imaginary circle of a predetermined radius having the attention point as a center the imaginary circle, and a plurality of second reference points corresponding to the first reference points outside the imaginary circle, each of the second reference points being set on a straight line connecting the attention point and the corresponding one of the first reference points, the water droplet detection unit being further programmed to detect edge information between the first reference points and second reference points, and the water droplet detection unit being further programmed to assess a circularity strength of the edge information, thereby detecting a water droplet attached to the photographic optical system, the circularity strength being determined as a ratio of a number of pairs of the corresponding first and second reference points between which the edge information was detected to a total number of the corresponding first and second reference points, and the water droplet attached to the photographic optical system being detected when the circularity strength is equal to or greater than a prescribed ratio.