An optical communication system includes a first communication device configured to transmit optical signals, and a second communication device configured to receive the optical signals. The first transmission device includes encoding circuit that configured to assign, to a plurality of bit strings, symbols each corresponding to a value of every one of the plurality of bit strings, the symbols being among a plurality of symbols in a constellation of a multi-level modulation scheme, convert values of bit strings, generate the second error correction code from a second bit string among the plurality of bit strings in every one of a plurality of periods, delay the first error correction code, and delay the second error correction code, wherein the encoding circuit uses the delayed first error correction code and the delayed second error correction code to convert a value of the second bit string.