PURPOSE: To attain the communication unless a fault takes place simultaneously in an active system and a standby system.

CONSTITUTION: When a fault takes place between a 1st stage repeater 2a and a 2nd stage repeater 2b in existence in a transmission line of an active system, for example, the 2nd repeater 2b sends the effect that a controller 4 detects a fault. In this case, the repeater 2b allows the controller 4 to switch an optical switch 9 to the position of a contact (b) thereby pulling in standby system transmission data via an optical branch coupler 8 of a correspondent repeater 2b'. Thus, the repeater 2b of the active system takes in normal data corresponding to fault information from the repeater 2b' of the standby system and sends the data to a repeater 2c of the next stage.