PURPOSE: To attain the extension of transmission distance and the improvement of a code error rate by performing frequency modulation on the output light of a light source synchronizing with the on-bits of light in accordance with three kinds of bits detected by a detection circuit.

CONSTITUTION: A frequency conversion circuit 16 drives a semiconductor laser 1 synchronizing with the on-bits of light in accordance with three kinds of bits; the rise bit, the fall bit, and the isolated bit of an information signal 3 detected by the detection circuit 7. The amplitude modulation of a drive current by a clock signal 4 generates frequency modulation on the output light 11 of the laser 1. A light modulator 2 is driven by the information signal 3, and transmits the light as a turned on and off optical signal 12 corresponding to the signal 3. At this time, phase shift matching between the signals 3 and 4 is performed by phase shifters 6 and 5 so as to apply the frequency modulation with three kinds of bits of the signal 3. The optical signal obtained in such a way is transmitted as the transmission signal of optical transmission equipment 13.