PROBLEM TO BE SOLVED: To provide a PMD(polarized wave mode dispersion) compensating method, that can compensate the PMD caused in a signal light over a wide range with high accuracy, and to provide a PMD compensation device. SOLUTION: The PMD compensation device 1A consists of a polarization control section 10, that sets a branch ratio γ or luminous intensity to two polarization modes as to a signal light received from a transmission line L, a variable PMD compensation section 11 that provides an optical delay difference ΔτC between the two polarization modes, a PMD monitor 13A that detects the intensity of a frequency f1 component of the signal light outputted from the variable PMD compensation section 11, a PMD monitor 13B that detects the intensity of a frequency f2 component (f2f1), and a control circuit 14 that applies feedback control to the polarization control section 10 and the variable PMD compensation section 11, so that each monitor intensity of the PMD monitors 13A, 13B lie within a maximum value converging range, depending on the uncertain width of the detection sensitivity.
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