PURPOSE: To prevent the vehicle running-off the road securely by a method wherein a transmitter/receiver is arranged widthwise on the outside of a body facing downward to measure the height of a road surface outside the body and an alarm sounds when the value exceeds the specified value.

CONSTITUTION: An ultrasonic wave transmitter/receiver 5 is mounted on the undersurface of a holder of a fender mirror 3 on the left of a body, namely opposite the driver's seat with the transmitting/receiving surface down the road surface. When the road surface G outside a wheel 6 is flat, a control circuit 10 does not output drive signal to an alarm 11 but when there is a ditch 12 or when a vehicle runs near the shoulder of a mountainous road or the like with the wheel 6 approaching the shoulder, a measured value signal obtained with the control circuit 10 exceeds a reference value signal and actuates an alarm 11 to warn an operator of the danger of the vehicle's running off the road.