In this study, aiming at clarifying the characteristics of air-to-ground radio wave propagation in mountainous areas, a transmission experiment was performed between a drone equipped with a transmitter and three receivers set up on the ground using a 920MHz band wireless system at Uchigatani forest, which is located in Yamato-cho, Gujo-shi, Gifu Prefecture. In the experiment, we simultaneously measured the received signal strength indicator (RSSI) and the drone's latitude, longitude, and height from the ground. Then, we verified whether the measured data has the line-of-sight between the transmitter and receivers using a geographic information system and analyzed characteristics of the RSSI, packet loss rate, and fading concerning the height from the ground and distance between the transmitter and receivers. The results showed that increasing the drone's altitude to 90m or more makes the link more stable and that the fading distribution in mountainous terrains is different from in other terrains.