To provide a digital radio transmission system capable of realizing stable transmission even on the occurrence of a change in a received state caused by a mobile relaying state or a change in weather.

In the digital radio transmission system provided with a plurality of modulation systems, a receiver is provided with a means for discriminating a proper modulation system for stable data transmission on the basis of the reception state such as a C/N in the receiver, and a radio means for transmitting a discrimination result to a transmitter, and the transmitter includes a radio means for receiving a discrimination result transmitted from the receiver and a means for revising a modulation system of the transmitter into a proper modulation system on the basis of the received discrimination result.