A transmission device includes: optical transmitters configured to transmit optical signals of different wavelengths; optical couplers configured to multiplexing the optical signals transmitted from the optical transmitters; and a wavelength selective switch configured to multiplex multiplexed optical signals obtained by multiplexing by the couplers, wherein optical signals, a wavelength interval between which is less than a predetermined value, are transmitted to separate optical couplers of the optical couplers, among the optical signals.