A droplet sensor includes an optical cover having an ellipsoid surface that is a portion of a spheroid, a light source disposed at or in proximity to a first focal point of the ellipsoid surface, and a light detector disposed at or in proximity to a second focal point of the ellipsoid surface. The ellipsoid surface includes an effective detection area configured to reflect light emitted by the light source toward the light detector, and an amount of light reflected by the effective detection area changes in accordance with adhesion of droplets on the ellipsoid surface. The optical cover includes a space having a hemispherical surface, the space being centered at the second focal point. The hemispherical surface includes a transmission scattering surface on a region that receives the light reflected by the effective detection area.