PURPOSE: To provide an optical repeater and optical transmission system so that a position where any fault is generated can be easily decided and thus a manager can speedily deal with it.

CONSTITUTION: An optical signal to be inputted to an optical fiber amplifier 11 is branched by a photocoupler 12, applied to a photodetector 14 and converted into an electric signal. An alarm signal generator 15 judges the level of the optical signal from the level of the electric signal applied from the photodetector 14 and monitors whether this level is higher than a prescribed value or not. On the other hand, when the level of the optical signal is lowered rather than the prescribed value, the alarm signal generator 15 sends an alarm signal in a prescribed pattern by driving a laser diode 16.