This paper describes a new line code which is suitable for a high-speed optical digital transmission system of more than 400 Mbits/s. This line code, named

code, is created by means of a complemental pulse which is inserted every

bits. This complemental pulse prevents long series of "l"\´s or "0"\´s. Power spectrum, jitter, and optical transmission characteristics as well as error monitoring performance are described on theoretical analyses and experimental results.