A relay device comprising: a non-regenerative relay system including: a first demultiplexing unit (5-1 to 5-3) that demultiplexes a reception signal to generate a demultiplexed signal in a band narrower than a band of the reception signal; and a first multiplexing unit (7-1 to 7-3) that multiplexes an input signal to generate a multiplexed signal, a second multiplexing unit that multiplexes an input signal to generate a multiplexed signal; a transmission antenna (11-1 to 11-3) that transmits the multiplexed signal generated by the first multiplexing unit; characterised by: a regenerative relay unit that carries out demodulation processing on the multiplexed signal generated by the second multiplexing unit and modulates an information sequence obtained by the demodulation processing to generate a modulated signal, a second demultiplexing unit that demultiplexes the modulated signal to generate a demultiplexed signal in a band narrower than the band of the reception signal; and a switch unit that inputs the demultiplexed signal generated by the first demultiplexing unit to the first multiplexing unit or the second multiplexing unit and inputs the demultiplexed signal generated by the second demultiplexing unit to the first multiplexing unit. a non-regenerative relay system including: a first demultiplexing unit (5-1 to 5-3) that demultiplexes a reception signal to generate a demultiplexed signal in a band narrower than a band of the reception signal; and a first multiplexing unit (7-1 to 7-3) that multiplexes an input signal to generate a multiplexed signal, a first demultiplexing unit (5-1 to 5-3) that demultiplexes a reception signal to generate a demultiplexed signal in a band narrower than a band of the reception signal; and a first multiplexing unit (7-1 to 7-3) that multiplexes an input signal to generate a multiplexed signal, a second multiplexing unit that multiplexes an input signal to generate a multiplexed signal; a transmission antenna (11-1 to 11-3) that transmits the multiplexed signal generated by the first multiplexing unit; characterised by: a regenerative relay unit that carries out demodulation processing on the multiplexed signal generated by the second multiplexing unit and modulates an information sequence obtained by the demodulation processing to generate a modulated signal, a second demultiplexing unit that demultiplexes the modulated signal to generate a demultiplexed signal in a band narrower than the band of the reception signal; and a switch unit that inputs the demultiplexed signal generated by the first demultiplexing unit to the first multiplexing unit or the second multiplexing unit and inputs the demultiplexed signal generated by the second demultiplexing unit to the first multiplexing unit. a regenerative relay unit that carries out demodulation processing on the multiplexed signal generated by the second multiplexing unit and modulates an information sequence obtained by the demodulation processing to generate a modulated signal, a second demultiplexing unit that demultiplexes the modulated signal to generate a demultiplexed signal in a band narrower than the band of the reception signal; and a switch unit that inputs the demultiplexed signal generated by the first demultiplexing unit to the first multiplexing unit or the second multiplexing unit and inputs the demultiplexed signal generated by the second demultiplexing unit to the first multiplexing unit.