Of a sensing part and a mounting part of an oil level sensor mounted to a sensor mounting opening at a bottom portion of an oil pan of an engine, an undersurface of the mounting part exposed to the outside of the oil pan is located above the lowest part of the oil pan by a distance, and the mounting part is disposed in a recessed part formed on an undersurface of the oil pan and is covered with a cover plate. Therefore, the weight of the engine does not directly act on the mounting part of the oil level sensor when the engine is directly placed on a floor surface. Further, even in a state in which the engine is mounted on a vehicle body, the mounting part of the oil level sensor can be also effectively protected at the time of colliding with a flying stone, riding on a curbstone, jacking up or the like.