The present invention provides a rear under view mirror system for vehicles. The rear under view mirror system includes a rear under view mirror unit having a semitransparent glass and an electrically switchable glass. When a vehicle moves forwards, the rear under view mirror unit does not protrude rearwards relative to a vehicle body and overlaps a rear windshield glass, thus providing a good appearance and preventing resistance to airflow, without hindering a driver from seeing the area behind the vehicle through the rear windshield glass. The rear under view mirror unit has a large area corresponding to that of the rear windshield glass, thus providing a sufficient view of the low area to the rear of the vehicle.