In this invention, an optical analog signal obtained such that light having a wavelength .lambda.1 is intensity-modulated or frequency-modulated with a modulation index m per channel and a chirping amount .lambda.s for 100% modulation is transmitted. An optical element connected to an outlet of a transmission medium having a polarization mode dispersion .tau. with respect to an optical signal is selected such that when the optical signal output from the optical element requires a predetermined composite second-order distortion C, a transmittance T.sub.0 of the optical element for the light having the wavelength .lambda.1 and a polarization dependency A of an insertion loss satisfy the following condition:20 log.sub.10 {(2.pi.c/.lambda.1.sup.2).multidot.n.multidot.(A/T.sub.0).multidot..tau..multidot.m.multidot..lambda.s}.ltoreq.C(where n is a number of beats occurring in the optical signal and c is the light velocity).