PURPOSE: To attain stable optical pulse train and long range and high speed transmission line by generating a signal the same as an input signal when a sudden input exists and allowing the titled transmitter to generate a signal when no input exists so as to drive a semiconductor laser.

CONSTITUTION: A drive signal generating circuit 1 uses an input signal presence or absence detecting signal from a control signal generating circuit 8, generates a signal equal to the input signal 10 while the input signal 10 is inputted suddenly and generates a pulse signal or a DC signal when no input signal 10 is inputted and inputs a drive signal 30 to a drive circuit 2. On the other hand, a part of the optical signal of a semiconductor laser 3 is fed back to the circuit 2 via an optical multiplexing device 4, a photodetector 5 and a level detector 6. Further, the optical signal of the semiconductor laser 3 driven by the output of the circuit 2 is outputted externally only when the signal 10 is inputted by an optical switch 7 controlled by the signal 20.