A wavelength-division multiplexing optical transmission system for providing a compensating-purpose dispersion D2 to a wavelength-division multiplexing optical signal to be transmitted through an optical transmission path from a transmitter terminal to a receiver terminal. The compensating-purpose dispersion D2 satisfies conditions that at any wavelength ".lamda." included in the transmission wavelength band, if dD1(.lamda.)/d.lamda..noteq.0 is established, then {dD1(.lamda.)/d.lamda.}.times.{dD2(.lamda.)/d.lamda.}<0 is also established, and if dD1(.lamda.)/d.lamda.=0 is established, then dD2(.lamda.)/d.lamda.=0 is also established, where D1 represents a dispersion generated in the wavelength-division multiplexing optical signal during when the wavelength-division multiplexing optical signal is transmitted through the transmission path from the transmitter terminal to the receiver terminal.