A radio transmission system includes a transmitter and a receiver. The transmitter includes: first and second antennas; a transmission circuit converting an signal into a radio signal corresponding to a transmission channel, and transmitting the radio signal via a transmission antenna; a receiving circuit measuring an electric field strength, for each channel, of a radio wave which a receiving antenna receives; a control circuit selecting first antenna as the receiving antenna, selecting a channel which includes the smallest strength as the transmission channel, selecting first antenna as the transmission antenna when the largest strength is equal to or smaller than a threshold value, and selecting second antenna as the transmission antenna when the largest strength is larger than the threshold value; and a switching circuit connecting the transmission circuit and the receiving circuit to the antennas, according to the selection by the control circuit.