A study is performed on the feasibility of DME-based RNAV system in Japan, where the usage of such an airborne equipment is not authorized. It is proposed here a navigation method that utilizes three DME stations (responders) and the airborne altimeter to obtain the positional information of an aircraft. Through computations, it is invesigated how the navigation system shows its effectiveness in Japan with her actual DME fascilities (counting a little more than a hundred). The HDOP is used as the evaluation parameter. Obtained results show that the DME-based navigation would exhibit adequate characteristics on the en-route area all over Japan and her vicinity; except Okinawa region where the geographical condition refuses to possess the favourable configuration of DME stations.