PURPOSE: To accurately synchronously transmit data by sending the clock signals of the same period from both terminal stations and obtaining the middle time of respective clock signal reception times in each repeater to use this time as the data measuring time.

CONSTITUTION: E/O converting circuits 21∼26 and O/E converting circuits 31∼36 are connected to repeating transmitters (#0∼#N) 1 and are coupled to an optical fiber 5 through optical multiplexer/demultiplexers 4. A clock signal 61 is sent to an outgoing system from the #0 equipment 1 with a certain time T as the period and is not only received as a clock signal 62 but also repeated and transmitted by the #1 equipment 1 and is received as clock signals 63 and 64 by following equipments 1. Meanwhile, the #N equipment 1 independently sends a clock signal 71 to an outgoing system with the certain time T as the period, and this signal is received as signals 71∼74 by equipments 1. Each equipment 1 obtains the middle time 8 of clock signal reception times of incoming and outgoing systems to accurately synchronously transmit data.