When a failure occurs in operational transmission multiplexing transponders 6-1 to 6-N, a wavelength multiplexing optical transmission system carries out selection switching control on N×1 optical switches 5-1 to 5-m and (N+1)×N optical switches 11-1 to 11-m to transmit a wavelength-multiplexed light signal by using reserve transmission and reception multiplexing transponders 6-R and 10-R, thereby sidestepping the failure.