PURPOSE: To ensure an effective elimination and reduction of the interference wave by making the carrier information signal mixed with the interference wave pass through the nonlinear amplifier and the band pass filter featuring the dispersion property and then amplifying the output and the information signal through the linear amplifier to obtain the difference signal.

CONSTITUTION: Carrier information signal X in which an intensive interference wave N(t)cos{ωO+(t)} mixed into information signal Qcos{ωOt+θs(t)} is supplied to nonlinear amplifier 2 and linear amplifier 3 featuring the dispersion property. The dispersion property shows a much sharper increase than output y'1 increment which is proportional to input X, while output y2 of the linear amplifier is proportional to input X. Then output B of amplifier 2 which contains the higher harmonic component is removed through band-pass filter 4 featuring the central angle frequency of angle frequency ωO of the carrier, and only signal D close to the carrier frequency is drawn out to be applied to differentiating circuit 5. At the same time, output C of amplifier 3 is also applied to circuit 5, and difference signal 2 between D and C is taken out of output terminal 6 in the form of the output.