A droplet sensor includes an optical cover having an elliptical surface forming a portion of a spheroid, a light source arranged at or in a vicinity of a first focal point of the elliptical surface, and a photodetector arranged at or in a vicinity of a second focal point of the elliptical surface. The elliptical surface includes an effective detection area configured to reflect light emitted from the light source towards the photodetector. An amount of light reflected from the effective detection area changes according to adhesion of a droplet on the effective detection area of the elliptical surface. The optical cover includes a hollow portion that is provided in a region inside the optical cover, and outside optical paths of the light emitted from the light source and reflected from the effective detection area toward the photodetector.