An optical transmission apparatus is arranged by: means for demultiplexing monitoring light for received wavelength-multiplexed signal light so as to detect optical intensity of the monitoring light; means for detecting optical intensity of wavelength-multiplexed signal light after the monitoring light has been demultiplexed therefrom; a gain controlling type optical amplifier for amplifying the wavelength-multiplexed signal light; an optical attenuating unit for adjusting optical intensity of the amplified wavelength-multiplexed signal light; and a monitoring control unit for controlling the gain controlling type optical amplifier in such a manner that the gain of the optical amplifier becomes constant, and for controlling an attenuating amount of the optical attenuating unit in such a manner that the optical intensity of the wavelength-multiplexed signal light becomes a predetermined target value.