The present invention relates to an apparatus for generating quasi equally spaced multi-channel frequencies using a frequency shifter. The optical multi-channel transmission system using OFDM or WDM can be provided with quasi equally spaced(unequally sequential spaced) frequencies on the basis of the functional relation among the frequency differences(n.sub.ij =n.sub.i -n.sub.j (i=j=1,2,3, . . . n)) of any two neighboring channels. Namely, after generating equally spaced frequencies by using a reference resonator, such as a Fiber Fabry Perot(FFP) resonator, each of the equally spaced frequencies is sequentially shifted by a frequency shifter, which generates and controls the quasi equally spaced(unequally sequential spaced) frequencies in order to effectively avoid crosstalk due to FFWM, and fully make use of the transmission bandwidth belonging to the low dispersion region of the optical fiber. the effect of the present invention will be apparent as follows: by easily generating and controlling quasi equally spaced frequencies having a functional relation between two adjacent channels, crosstalk due to Fiber Four Wave Mixing(FFWM) is effectively avoided, and the transmission bandwidth in the region of the low dispersion is fully utilized. Furthermore, the compatibility is easily accomplished.