m when the testing sine wave transmitted from repeater 10 1 is received by repeater 10 m optical receiver 14 provided in repeater 10 m converts the testing sine wave inputted thereto into an electric signal which is inputted to frequency characteristic measuring instrument 16 and frequency characteristic equalizer 13 consequently frequency characteristic measuring instrument 16 detects the level of the testing sine wave inputted thereto then the level of the testing sine wave detected by frequency characteristic measuring instrument 16 is inputted to frequency characteristic discriminator 17 in which it is compared with a reference value set in advance to discriminate whether the frequency characteristic of the testing sine wave after conversion into the electric signal by the optical receiver 14 should be raised or dropped then based on a result of the discrimination control signal b is outputted from frequency characteristic discriminator 17 when control signal b is outputted from frequency characteristic discriminator 17 it is inputted to frequency characteristic equalizer 13 by which adjustment of the frequency characteristic of the testing sine wave inputted from optical receiver 14 is performed in response to control signal b when the adjustment is completed the testing sine wave whose frequency characteristic has been adjusted and control completion signal c are outputted from frequency characteristic equalizer 13 the testing sine wave outputted from frequency characteristic equalizer 13 is converted into an optical signal by optical transmitter 15 and transmitted to a repeater connected in the following stage when control completion signal c is outputted from frequency characteristic equalizer 13 frequency designation signal a with which the frequency.