PURPOSE: To accurately confirm the allowance behind an automobile body, by a method wherein the distance up to an obstruction detected by an obstruction detector is digitally displayed, for example, on an automobile speed display part being one of meter display parts when a car is to be backed.

CONSTITUTION: When the operation signal of a back switch 5 operated when a gearbox is shifted to reverse is inputted, a CPU 1 emits a signal operating a back sonar sensor A and the back sonar sensor A intermittently transmits an ultrasonic wave to the rear of a car body through a transmitting circuit 3 and an interface 4. When the ultrasonic wave impinges against an obstruction to be reflected therefrom, the reflected wave is received by the sensor A to be inputted to CPU 1 through the interface 4 and the CPU 1 operates the distance up to the obstruction from the time ranging from transmission to reception. When the operation signal of the switch 5 is inputted and the sensor A beings to operate and the distance up to the obstruction becomes a set value or less, the CPU 1 changes over the display of a digital meter part C, for example, the car speed display part 2 thereof from inherent speed display to the distance display 2' up to the obstruction to digitally display the distance up to the rear obstruction.