A relay device that relays a signal in which a plurality of signals having respectively different frequencies are multiplexed between a first communication device on a transmission side and a second communication device on a reception side, the relay device comprising: a demultiplexing unit adapted to demultiplex a signal received from the first communication device to a plurality of frequencies; a relay-method decision unit adapted to decide which relay method is performed on a signal having been demultiplexed by the demultiplexing unit among a plurality of relay methods including regenerative relay and transparent relay; a signal regeneration unit (15, 16, 17, 18) adapted to perform demodulation, decoding, coding, and modulation on a signal decided to undergo the regenerative relay by the relay-method decision unit; and a multiplexing unit adapted to multiplex a signal decided to undergo the transparent relay by the relay-method decision unit and a signal regenerated by the signal regeneration unit, wherein the relay-method decision unit includes a line-quality measurement unit adapted to measure a line quality of each of demultiplexed signals, which are the signals having been demultiplexed, and a decision unit adapted to decide a relay method of each of the demultiplexed signals based on the line quality, characterized in that the decision unit has a first threshold and a second threshold that is smaller than the first threshold, and the decision unit is adapted to decide: to perform the transparent relay on a signal having a line quality equal to or larger than the first threshold among the demultiplexed signals; to perform the regenerative relay on a signal having a line quality less than the first threshold and equal to or larger than the second threshold among the demultiplexed signals, and not to relay and to discard a signal having a line quality less than the second threshold among the demultiplexed signals. a demultiplexing unit adapted to demultiplex a signal received from the first communication device to a plurality of frequencies; a relay-method decision unit adapted to decide which relay method is performed on a signal having been demultiplexed by the demultiplexing unit among a plurality of relay methods including regenerative relay and transparent relay; a signal regeneration unit (15, 16, 17, 18) adapted to perform demodulation, decoding, coding, and modulation on a signal decided to undergo the regenerative relay by the relay-method decision unit; and a multiplexing unit adapted to multiplex a signal decided to undergo the transparent relay by the relay-method decision unit and a signal regenerated by the signal regeneration unit, wherein a line-quality measurement unit adapted to measure a line quality of each of demultiplexed signals, which are the signals having been demultiplexed, and a decision unit adapted to decide a relay method of each of the demultiplexed signals based on the line quality, characterized in that the decision unit has a first threshold and a second threshold that is smaller than the first threshold, and the decision unit is adapted to decide: to perform the transparent relay on a signal having a line quality equal to or larger than the first threshold among the demultiplexed signals; to perform the regenerative relay on a signal having a line quality less than the first threshold and equal to or larger than the second threshold among the demultiplexed signals, and not to relay and to discard a signal having a line quality less than the second threshold among the demultiplexed signals.