To make a serial signal to be transmitted into the signal that an mBnB code is multiplexed for every word and to completely exhibit the mark ratio improving effect that the code is intrinsically provided.

The encoding circuits 11 to 14 on a transmission side perform 4B5B encodings for parallel signals of 4-bit width and output parallel signals of 5-bit width. An interleave circuit 100 interleaves 4 parallel signals of 5-bit width, the bit multiplexings of two stages are performed for the parallel signals by first and second parallel/serial conversion circuits 21 to 24 and 30 and the signals are made serial signals. Next, optical transmissions are performed by an optical transmitter 40 and an optical receiver 60, the bit separations of two stages are performed by first and second serial/parallel conversion circuits 70 and 81 to 84 on a reception side, the parallel signals are made the parallel signals of 20-bit width, the signals are deinterleaved by a deinterleave circuit 110 and the signals are made four parallel signals of 5-bit width. Decoding circuits 91 to 94 decode the parallel signals of 5-bit width into 5B4B and output the parallel signals of 4-bit width.