In this study, a method of shortening the minimum detectable distance of an ultrasonic sensor for the distance measurement of automobiles was developed. It is necessary to overcome the limitations of the minimum detectable distance, which is induced by the residual vibration of an elastic disc in an ultrasonic sensor. The purpose of this study is to design a waveguide to shorten the minimum detectable distance and to improve the performance of proximal measurement by coupling the waveguide with an ultrasonic sensor. We simulated the ultrasound propagation in the designed waveguide through finite element analysis. The analytical results showed clear signals of transmitted and received ultrasound in a wedged-wall waveguide instead of a flat-wall waveguide. Experimental tests were performed to verify the analytical results. The waveguides of the flat and wedged walls were fabricated using a 3D printer and applied during the experiments. The results showed that the ultrasonic sensor coupled with the wedged-wall waveguide measured the proximal distance reasonably.