Raindrop sensor having plano-convex lens

This application is based upon and claims the benefit of Japanese Patent Applications No. 11-242096 filed on Aug. 27, 1999, the contents of which are incorporated herein by reference. A raindrop sensor has a prism body, incoming radiation side plano-convex lens portions each provided on inclined surfaces of the prism body for receiving light from light-emitting elements, and outgoing radiation side plano-convex lens portions provided on inclined surfaces of the prism body for emitting light toward a light-receiving element. The plano-convex lens portions are configured to cause a relatively small number of light-receiving element(s) to receive light from a number of outgoing radiation side plano-convex lens portions. Such a raindrop sensor can be made to have a more structure while employing a widened detection region.