An illuminance sensor for detecting brightness of outside a vehicle having a windshield includes first and second light receiving elements and a light emitting element for emitting light to the windshield. The first light receiving element outputs a first signal corresponding to outside light. The light emitted from the light emitting element is reflected at an outside surface of the windshield. The second light receiving element outputs a second signal corresponding to the reflected light. The outside light passes a first region of the windshield through which the light emitted from the light emitting element passes or a second region of the windshield through which the reflected light passes. The first signal is corrected based on the second signal to detect the brightness.