An optical repeater in a mode-division-multiplexing (MDM) optical transmission system prevents a signal-to-noise ratio (SNR) from deteriorating due to a loss of a mode demultiplexer. The optical repeater in the MDM optical transmission system is configured to include at least one multimode optical amplifier, a mode demultiplexer, a mode multiplexer, and variable optical attenuators. One of the multimode optical amplifiers is disposed on the input side of the mode demultiplexer.