To provide an optical communication apparatus in which the nonlinear degradation of a wavelength multiplexed signal in the nonlinear degradation factor member causing the nonlinear degradation is reduced, to provide an optical transmission system using the same, and a method for reducing the nonlinear degradation.

There are provided a wavelength interval enlarging portion 51 for enlarging a wavelength interval of an input wavelength multiplexed signal, a nonlinear degradation incidental processing portion 52 for applying optional processing involving the nonlinear degradation to the wavelength multiplexed signal whose wavelength interval is enlarged, and a wavelength interval recovering portion 53 for recovering the wavelength interval of the wavelength multiplexed signal to which the optional processing is applied to the original wavelength interval.