PURPOSE: To obtain the transmission codes useful in the optical communication by dividing equally the bit conditions of NRZ code as the input code or RZ code into two binary conditions and converting the code in accordance with the bit condition of the input code.

CONSTITUTION: NRZ code or RZ code is inputted 1 and converted into the aimed codes 3 by the code converter 2. The converting operation in the converter 2 is carried out in accordance with the illustrative code converting figure; a, b and complements -a∼-c show the condition of the input code and the conversion code with respect to the input code in accordance with the condition of the input code and the immediately before the converted code is determined. For example, if a=b=c=1, the just before the conversion code held in the converter 2 is 00, when the input code is a, it is converted into 11 and the input code is -a, it is converted into 01. When immediately before the converted code is 11, it is converted into 00 or 01, and when the code is 01, it is converted into 00 or 01 and the input bit is divided and converted into two binary conditions. Since the converted code has many 0, it results in the long life of the light emitting means of the optical communication.