PURPOSE: To transmit an optical output corresponding to a transmission path by automatically adjusting the optical output of an optical transmission section with respect to the automatic optical output adjustment system of the optical transmission section using an LD element for a light source.

CONSTITUTION: The optical transmission section is provided with a voltage comparison stabilizing circuit 17, which controls the bias if an LD element depending of the error output between a detected value of a rear side light of the LD element 1 and a threshold level so as to make the optical output of the LD element stable. In this case, a comparator 12 is provided to compare a detected optical output level of the LD element sent to an optical transmission line 4 with a setting output level to detect the error and the threshold level of the voltage comparison stabilizing circuit 17 is controlled depending on the detected error to make the optical output level on the optical transmission line constant.