1. A vehicle backup detector that indicates a distance from an obstruction comprising multiple ultrasonic transducers emitting and receiving ultrasonic waves, and each ultrasonic transducer outputting a signal when a reflected ultrasonic wave is received; a central processing unit being connected to the ultrasonic transducers, switching the ultrasonic transducers between emitting and receiving modes, defining multiple sensing coordinates in a sensing area in which the ultrasonic waves are transmitted, calculating a distance between the ultrasonic transducers and the obstruction, calculating a position of the obstruction and storing the position in corresponding coordinates in the sensing area; memory being connected to the central processing unit and being divided into multiple storing fields corresponding to the sensing coordinates in the sensing area to allow the central processing unit to store obstruction data respectively in the corresponding storing fields based on the position of the obstruction; a video camera capturing images behind the vehicle and outputting image signals; a graphics superimposing module being connected to the central processing unit and the video camera and superimposing graphical images representing ultrasonic waves on images recorded by the video camera where the storing fields of the memory do not contain obstruction data; and a display device being connected to the video camera and the graphics superimposing module to display images behind the vehicle with the ultrasonic wave images.