To easily judge a cause when a reception signal level drops due to the drop of an altitude in a signal transmission system where the position and the altitude of a mobile object are monitored by a display device based on position data transmitted from the flying mobile object.

A signal transmission system has a transmission device modulating the signal of a micro frequency band by a video signal and a sound signal and transmitting a modulation signal, a reception device receiving the transmitted modulation signal by a rotary antenna and a controller controlling the rotary antenna. The system has a means for accumulating altitude data on an area containing the position of the transmission device and that of the rotary antenna, a means for accumulating position information and altitude information on the rotary antenna, a means for measuring the position information and altitude information on the transmission device and a means for accumulating measured position information and altitude information. An altitude cross section drawing between the transmission device position and the rotary antenna position is displayed based on the accumulated altitude data, position information and altitude information.