A sensing system using sonar ring for mobile robot capable of rapid detection and avoidance of obstacles has been developed. In the system, a method of firing all ultrasonic sensors on a sonar ring simultaneously is used and to eliminate error data due to crosstalks in the simultaneous firing, a filtering method based on pattern matching using neural network is used. To show the advantage of this method, a prototype of sensing system with 24 sonar sensors is fabricated. By installing the system to mobile robot, a scanning rate up to 50 Hz of panoramic detection is achieved. The experimental results showed that a rapid navigation of mobile robot in a real environment becomes possible. This paper describes the basic concept, the method for sensing and avoidance of obstacles, configuration of the developed sensor system and experimental results of obstacle detection and avoidance.