An optical fiber transmission system (100, 200) that performs mode division multiplexed transmission, the system (100, 200) comprising: a transmission line including an optical fiber (10, 20) including at least one core or core group and one cladding surrounding the at least one core or core group, the optical fiber (10, 20) being configured to allow M degenerate spatial mode groups to propagate in the at least one core or core group, with M being greater than or equal to three; at least two transmitters (60A to 60D) disposed adjacent to an input end of the transmission line and configured to output different optical signals in the spatial mode groups; at least two receivers (70A to 70D) disposed adjacent to an output end of the transmission line in such a manner as to correspond to the at least two transmitters (60A to 60D), the receivers (70A to 70D) each being configured to receive an optical signal being output by a corresponding one of the transmitters (60A to 60D) and propagated in the spatial mode groups through the transmission line; input-end crosstalk reducing means for limiting, on the side of the input end of the transmission line, input of an optical signal to not use at least one spatial mode group having a propagation constant larger than the smallest of propagation constants of respective spatial mode groups used for signal transmission, the at least one spatial mode group being not used for signal transmission, wherein in the at least one core or core group, the optical fiber (10, 20) transmission system (100, 200) transmits signals using (M - 1) or less of the M spatial mode groups, and the system (100, 200) further comprising a MIMO configuration configured to not perform crosstalk compensation between non-degenerate modes and perform inter-mode crosstalk compensation for each of the degenerate spatial mode groups by treating the plurality of degenerate spatial mode groups as one spatial channel. a transmission line including an optical fiber (10, 20) including at least one core or core group and one cladding surrounding the at least one core or core group, the optical fiber (10, 20) being configured to allow M degenerate spatial mode groups to propagate in the at least one core or core group, with M being greater than or equal to three; at least two transmitters (60A to 60D) disposed adjacent to an input end of the transmission line and configured to output different optical signals in the spatial mode groups; at least two receivers (70A to 70D) disposed adjacent to an output end of the transmission line in such a manner as to correspond to the at least two transmitters (60A to 60D), the receivers (70A to 70D) each being configured to receive an optical signal being output by a corresponding one of the transmitters (60A to 60D) and propagated in the spatial mode groups through the transmission line; input-end crosstalk reducing means for limiting, on the side of the input end of the transmission line, input of an optical signal to not use at least one spatial mode group having a propagation constant larger than the smallest of propagation constants of respective spatial mode groups used for signal transmission, the at least one spatial mode group being not used for signal transmission, wherein in the at least one core or core group, the optical fiber (10, 20) transmission system (100, 200) transmits signals using (M - 1) or less of the M spatial mode groups, and the system (100, 200) further comprising a MIMO configuration configured to not perform crosstalk compensation between non-degenerate modes and perform inter-mode crosstalk compensation for each of the degenerate spatial mode groups by treating the plurality of degenerate spatial mode groups as one spatial channel.