An optical communication system (301; 302) configured to perform digital coherent transmission, the optical communication system (301; 302) comprising: N, N being an integer of 2 or greater, transmission channels configured to transmit optical signals obtained by modulating light beams with pieces of transmission data; N receivers connected to the N transmission channels in a one-to-one manner and configured to receive the optical signals transmitted through the corresponding N transmission channels by performing coherent detection with a single local oscillator light source; characterized by: a signal processing unit connected to a receiver of the N receivers connected to a shortest transmission channel and configured to acquire a transfer function for removing signal distortion of an optical signal of the optical signals propagated through the shortest transmission channel, the shortest transmission channel being a transmission channel with a shortest transmission delay time among the N transmission channels; and N - 1 demodulation units connected, in a one-to-one manner, to receivers of the N receivers connected, in a one-to-one manner, to other transmission channels which are other than the shortest transmission channel among the N transmission channels and configured to remove signal distortions of optical signals propagated through the corresponding other transmission channels by using the transfer function and demodulate the pieces of transmission data. N, N being an integer of 2 or greater, transmission channels configured to transmit optical signals obtained by modulating light beams with pieces of transmission data; N receivers connected to the N transmission channels in a one-to-one manner and configured to receive the optical signals transmitted through the corresponding N transmission channels by performing coherent detection with a single local oscillator light source; characterized by: a signal processing unit connected to a receiver of the N receivers connected to a shortest transmission channel and configured to acquire a transfer function for removing signal distortion of an optical signal of the optical signals propagated through the shortest transmission channel, the shortest transmission channel being a transmission channel with a shortest transmission delay time among the N transmission channels; and N - 1 demodulation units connected, in a one-to-one manner, to receivers of the N receivers connected, in a one-to-one manner, to other transmission channels which are other than the shortest transmission channel among the N transmission channels and configured to remove signal distortions of optical signals propagated through the corresponding other transmission channels by using the transfer function and demodulate the pieces of transmission data. a signal processing unit connected to a receiver of the N receivers connected to a shortest transmission channel and configured to acquire a transfer function for removing signal distortion of an optical signal of the optical signals propagated through the shortest transmission channel, the shortest transmission channel being a transmission channel with a shortest transmission delay time among the N transmission channels; and N - 1 demodulation units connected, in a one-to-one manner, to receivers of the N receivers connected, in a one-to-one manner, to other transmission channels which are other than the shortest transmission channel among the N transmission channels and configured to remove signal distortions of optical signals propagated through the corresponding other transmission channels by using the transfer function and demodulate the pieces of transmission data.