Part of output light of an LD 11 issuing a signal light 10 depending on a driving current is received by a photo diode 13, and the current corresponding to the reception power is converted into a voltage in a current-voltage converting circuit 14, and an average voltage V1 is detected in an average voltage detecting circuit 15. The average voltage V1 and a reference voltage V2 depending on a mark rate issued from a reference voltage control circuit 16 are entered in an optical output control circuit 17, and the driving current to be supplied into the LD 11 is controlled by a laser driving circuit 19 through a driving current control circuit 18 so that the LD 11 may not pass any current exceeding a preset value. At the time of response by optical output cut-off or its cancellation, by controlling the average voltage V1 and reference voltage V2 by the excessive output preventive circuit 41, generation of excessive optical output is prevented. As a result, an automatic power control circuit of the optical transmitter can emit an output light within the standard from the laser diode even at the time of transient response by optical output cut-off or cancellation of optical output cut-off.