The recent trend toward large scale and high speed ships and rapid increase in density on the sea indicates that the danger of collision of ships has become greater. The issue of reduction of collision risk has been very topical in these years. In 1973 Japan Radio Co. developed an new anti-collision system. The sea tests we (The Institute for sea Training and Japan Radio Co.) made in Coastal waters in Japan showed that this system has very good performance. Since the true speed vectors and the relative speed vectors of targets are superposed on the relative PPI presentation, navigators can easily determine what collision rule should be applied.