An optical communication system suitable for full duplex optical communication between a first and a second optical transceiver units. connected with each other by a single-core optical fiber. The first and second transceiver units may have different types of light emitting elements emitting light having different loss rates in the single-core optical fiber. The system enables stable full duplex communication without setting up large scale or costly transmission units by relatively reducing the optical crosstalk involved. The first optical transceiver unit provides transmission light to be coupled with the single-core optical fiber with the intensity in the range of TAmin TAmax. The amount of light emitted from the light emitting element of the optical transceiver unit to be coupled with the single-core optical fiber is in the range TBmin TBmax, which amount may be adjusted by a light emission adjusting unit provided in the optical transceiver unit.