PURPOSE: To eliminate the need of optical switches and to improve the reliability of the overall system, by constituting terminals including bypasses with electric transmission lines, in a transmission system where a lot of terminals are connected in loop with optical transmission lines.

CONSTITUTION: A lot of terminals 6 are connected in loop with an optical transmission line 5. Each terminal 6 is connected to incoming and outgoing optical transmission lines via an electric transmission line including photoelectric and electrooptic converters 8, 9 and a bypass switch 10, and if the terminal 6 is failed, the bypass switch 10 is activated to electrically couple the incoming and outgoing optical transmission lines. Through the constitution as stated above, no optical switch is required for terminals and various disadvantages of the transmission system based on the presence of the optical switches can be eliminated.