In a wavelength-division-multiplexing optical transmission system in which
optical-node apparatuses that perform relay transmission of
wavelength-division-multiplexed light are located at specified nodes in a
main optical transmission path, an optical amplifier switches level
control to constant-gain control when there is a notification of a change
in the number of multiplexed wavelengths, and then after a specified
amount of time restarts level control so that the level becomes a target
level that corresponds to the actual number of wavelengths. When doing
this, an optical-transmission-system-control unit controls the
optical-transmission system so that the equation T