An optical transmission device capable of preventing deterioration in the transmission quality of a signal beam. A wavelength-multiplexed signal beam from a transmission path is split by a splitter, and the split signal beams are input to first and second optical paths, respectively. An add/drop unit for adding/dropping a wavelength-multiplexed signal beam is removably insertable in the second optical path. A switch outputs the wavelength-multiplexed signal beam, received from one of the first and second optical paths, to an optical amplifier. The optical amplifier amplifies the wavelength-multiplexed signal beam output from the switch and outputs the amplified signal beam to a subsequent circuit. A supervisory controller collects alarm information from the optical transmission device as well as from other optical transmission devices, and controls the switch in accordance with the collected alarm information such that the wavelength-multiplexed signal beam from the second optical path is input to the optical amplifier.