An on-board optical sensor apparatus includes an optical sensor, an optical sensor cover, and a controller. The optical sensor has a lens. The optical sensor cover includes a holder and a washer nozzle. The holder holds the optical sensor. The washer nozzle performs a washing operation to wash a lens surface of the lens of the optical sensor or a glass surface of a cover glass located facing the lens by spraying a washer fluid, supplied from a washer fluid tank, onto the lens surface or the glass surface. The controller controls the washing operation of the washer nozzle.