The present invention relates to an optical level control method for use in an
optical transmission system. In the system, a WDM terminal comprises a multiplexing/demultiplexing
unit, an amplifier, an up-direction OSC light transmitting/receiving unit, an up-direction
multiplexer, an up-direction branching unit, an amplifier and an APR control unit,
and a repeater comprises an up-direction branching unit, an OSC light transmitting/receiving
unit, an amplifier, an up-direction multiplexer, a down-direction branching unit,
an amplifier, a down-direction multiplexer and an APR control unit. In this configuration,
a flexible optical output level control in connection with variation in WDM light
level, a selective value irrespective of occurrence of an error stemming from the
passage of time and avoidable malfunctions are respectively capable. In addition,
a trouble retrieving operator can eliminate the possibility of being exposed to
the WDM light.