A low-cost configuration of, and at the same time to control the variable dispersion compensator at a high speed in a variable dispersion compensator for compensating the wavelength dependent accumulated dispersion resulting from the wavelength dependency of the transmission fiber and fixed dispersion compensator in a long-distance high-speed WDM transmission system. In order to achieve the object mentioned above, the wavelength dependent representative characteristic of the transmission fibers 4-1 . . . n, and the wavelength dependent representative characteristic of the DCFs 13-1 . . . n are recorded and maintained in advance in the dispersion control circuit 5-1 . . . n of the variable dispersion compensator 16, and based on the input of dispersion amount at the representative length of the transmission fiber and the fiber length, the dispersion amount at the representative wavelength of the DCF, the accumulated dispersion amount is computed from the wavelength dependent representative characteristic recorded and maintained in advance, and the dispersion amount of the variable dispersion compensator 16 is determined by taking this as the reference.