An optical transmission system includes an optical transmitting unit, a multi-core optical fiber, an optical multiplexing unit, an optical demultiplexing unit, and an optical receiving unit. The optical transmitting unit outputs optical signals having a plurality of wavelengths included in an operation wavelength band. The multi-core optical fiber is connected to the optical transmitting unit and has a plurality of core portions. The optical multiplexing unit multiplexes the optical signals to the core portions. The optical demultiplexing unit demultiplexes the optical signals transmitted through the multi-core optical fiber. The optical receiving unit receives the optical signals demultiplexed by the optical demultiplexing unit, and the optical signals having different wavelengths from each other are input to different core portions from each other.