In order to avoid the loss of separation or collision risk among unmanned aircrafts as well as manned aircrafts, we have developed a prototype system where the aircrafts autonomously exchange their location information obtained through the signal of the Global Positioning System (GPS) using device-to-device (D2D) communications. Based on the developed system, we have conducted an experiment that demonstrates its successful operation for maintaining the targeted separation between the drone and manned helicopter. This paper will report the details of our experimental results as well as describe our finding of the requirement in D2D communications so as to guarantee the sufficient separation among the aircrafts.