Abstract: There is provided a coding/decoding system of bit insertion manipulation line code, which has small increase in bit rate and simple configuration and minimizes DC baseline fluctuations by properly coupling the advantages of mBnB code and bit insertion code in a high-speed optical transmission system using AC coupling. The encoding part of the system inserts a bit for minimizing a disparity of m+1 bit block when a disparity with respect to m-bit input information is a minimum value that the m+1 bit block may have, and inserts ‘0’ when it is not, performing pre-coding. Then, the encoding part manipulates a part of the bits of the pre-coded block to make its disparity have the minimum value, compares the disparity of the pre-coded block with a disparity accumulated, and inverts or non-inverts the block, thus carrying out coding.