on the received waveforms it will be seen that a code error rate of 1 u00d7 10 20 can be attained after 9000 km transmission while in this embodiment the accumulated wavelength dispersion in the optical fiber for each path of 360 km is compensated for by inserting the dispersion medium 4 in the optical fiber at the intermediate point thereof the dispersion medium 4 need not always be provided at such an intermediate point of the optical fiber but may be disposed at an arbitrary place in a predetermined section fig1 illustrates a third embodiment of the present invention reference numeral 1 denotes a 20 gbit s lightwave signal transmitting device 2 an optical fiber for transmission use 3 optical amplifying repeaters 4 dispersion media for accumulated wavelength dispersion compensating use and 5 a lightwave signal receiving device in this embodiment the lightwave signal transmitting device 1 includes the dispersion compensating medium 4 having a dispersion value of 36 ps nm and is set so that the accumulated dispersion by the optical fiber for transmission use starts at a value other than zero this embodiment is identical in construction with embodiment 1 except the above even if the output waveform of the optical transmitting device is modified by passing it through the dispersion medium prior to transmission the lightwave signal can stably be transmitted fig1 shows the relationship of the accumulated wavelength dispersion value to the distance of transmission in embodiment 3 fig1 a and b show received waveforms obtained by computer simulations carried out on this embodiment it will be seen from a calculation.