An occupant monitoring device images a driver in a vehicle by an imager, projects light toward a driver seat side by an illuminator, controls operation of the imager and the illuminator by a controller, monitors the driver, based on a captured image by the imager, and outputs a monitoring result. A main board on which the controller is mounted is contained in a housing, and a sub-board is held by a lid body configured to close an opening of the housing. The main board and the sub-board are electrically connected so that the imager and the illuminator mounted on the sub-board face the driver seat side. A water preventing cover covers the sub-board, the lid body, and the housing from the driver seat side, the water preventing cover having a transmitting portion configured to transmit projected light from the illuminator and reflected light from the driver.