A 
wavelength-division multiplexed optical 
transmission system to keep the correlation of 
data patterns among 
wavelength channels to the low level, preventing large XPM and XGM from occurring when the correlation is strong, and assuring a stable transmitting quality. The 
wavelength-division multiplexed optical 
transmission system for transmitting an optical 
signal constructed by a frame has any of (1) a frame phase changing circuit for mutually differing transmitting frame phases between at least two or more 
wavelength channels among a wavelength channel group transmitted through the same 
optical fiber transmitting line 
system, or (2) a scrambling circuit using a data scrambler based on a pseudo 
random pattern for mutually differing scrambling patterns between two or more 
wavelength channels among a wavelength channel group transmitted through the same 
optical fiber transmitting line, or (3) a 
dummy data generating circuit for mutually differing 
invalid data patterns between at least two or more 
wavelength channels in a wavelength channel group transmitted through the same 
optical fiber transmitting line when transmitting meaningless data.