A computer for controlling a pre-emphasis process is provided at end terminal stations which are the terminal stations of a trunk. When a pre-emphasis process is performed to an optical signal directing from a terminal station to another terminal station, the computer of the terminal station controls the process. First, the terminal station causes another terminal station to transmit the spectrum information of the thus-received optical signal, and receives the information. The terminal station obtains the spectrum information of its own received optical signal. The terminal station arranges data for each transmission source of the received optical signal, and calculates the amount of a pre-emphasis process to be performed at a transmission side. The terminal station transmits pre-emphasis execution instructions to the other terminal stations according to the thus-calculated results.