Ultrasonic rangefinders have already been used successfully for indoor mobile robots. However, in an outdoor environment, the sensor should be robust against noise from vehicle engines and other sound sources. The robust sensor has already been proposed, but it requires a high-performance processor. In this paper the authors propose a new ultrasonic ranging sensor which is simple and low-cost. This sonar is robust in noisy environments. A RTZ (return to zero) signal having a certain time duration modulated with a 40 KHz carrier is transmitted from a transducer, then cross-correlation is calculated between the transmitted wave form and the demodulated received wave form. If objects are present the calculated cross-correlation has relatively large value. The authors can measure the range from the sensor to the objects by the time interval between the starting time of the transmitted signal and the time which has the peak value. The experimental results show the robustness of this sensor against the impulse noises. Furthermore, the authors propose that several sonar sensors can work at the same time without mutual interference, when the different time duration signals are used as the transmitting signals.