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, 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.