An optical transmission system, comprising: an optical nonlinear element, having nonlinear input/output characteristics, for reproducing an optical signal; a power detector (30, 40, 50) for photoelectrically converting an optical signal reproduced by said optical nonlinear element to detect a power of a specific frequency component from an obtained electrical signal; and a variable amplifier for amplifying the optical signal input to said optical nonlinear element, or a variable attenuator for attenuating the optical signal input to said optical nonlinear element, in response to a detection result by said power detector (30, 40, 50); characterised in that: said power detector (30, 40, 50) is operable to detect a power of a bit rate frequency component of the inputted optical signal and comprises: a photo detector for converting the optical signal reproduced by said optical nonlinear element to the electrical signal; a filter for retrieving the specific frequency component from the electrical signal obtained by said photo detector; and a power measuring device for measuring the power of the specific frequency component retrieved by said filter; and in that the system further comprises a control circuit for controlling said variable amplifier, or variable attenuator as the case may be, such that the power of the bit rate frequency component is minimised, based on a detection result by said power measuring device. an optical nonlinear element, having nonlinear input/output characteristics, for reproducing an optical signal; a power detector (30, 40, 50) for photoelectrically converting an optical signal reproduced by said optical nonlinear element to detect a power of a specific frequency component from an obtained electrical signal; and a variable amplifier for amplifying the optical signal input to said optical nonlinear element, or a variable attenuator for attenuating the optical signal input to said optical nonlinear element, in response to a detection result by said power detector (30, 40, 50); characterised in that: said power detector (30, 40, 50) is operable to detect a power of a bit rate frequency component of the inputted optical signal and comprises: a photo detector for converting the optical signal reproduced by said optical nonlinear element to the electrical signal; a filter for retrieving the specific frequency component from the electrical signal obtained by said photo detector; and a power measuring device for measuring the power of the specific frequency component retrieved by said filter; and in that the system further comprises a control circuit for controlling said variable amplifier, or variable attenuator as the case may be, such that the power of the bit rate frequency component is minimised, based on a detection result by said power measuring device. said power detector (30, 40, 50) is operable to detect a power of a bit rate frequency component of the inputted optical signal and comprises: a photo detector for converting the optical signal reproduced by said optical nonlinear element to the electrical signal; a filter for retrieving the specific frequency component from the electrical signal obtained by said photo detector; and a power measuring device for measuring the power of the specific frequency component retrieved by said filter; and in that the system further comprises a control circuit for controlling said variable amplifier, or variable attenuator as the case may be, such that the power of the bit rate frequency component is minimised, based on a detection result by said power measuring device. a photo detector for converting the optical signal reproduced by said optical nonlinear element to the electrical signal; a filter for retrieving the specific frequency component from the electrical signal obtained by said photo detector; and a power measuring device for measuring the power of the specific frequency component retrieved by said filter; and in that the system further comprises a control circuit for controlling said variable amplifier, or variable attenuator as the case may be, such that the power of the bit rate frequency component is minimised, based on a detection result by said power measuring device.