A transmission system includes a plurality of nodes in which respective adjacent nodes are coupled by a first kind of optical fiber compatible with light in a first wavelength band or a second kind of optical fiber compatible with light in a second wavelength band, wherein each of the plurality of nodes includes a transmitting node that generates a wavelength-multiplexed optical signal in the first wavelength band by carrying out wavelength multiplexing of a plurality of optical signals and transmits the wavelength-multiplexed optical signal, a receiving node that demultiplexes the plurality of optical signals from the wavelength-multiplexed optical signal and receives the plurality of optical signals, and one or more relay nodes that relay the wavelength-multiplexed optical signal from the transmitting node to the receiving node through the first kind or the second kind of optical fiber.