PURPOSE: To trace the frequency of a light of a local oscillation semiconductor laser with the frequency of an optical signal from a transmitter and to obtain an original signal stably by transmitting a carrier signal even in the absence of the original signal.

CONSTITUTION: A transmitter 100 multiplexes a carrier signal 400 onto an original signal 300, applies amplitude modulation to the original signal 300 by using the carrier signal 400 and the resulting signal is fed to the drive current of a semiconductor laser 1 to apply frequency modulation to the frequency of the light of the semiconductor laser 1. A receiver 200 detects a beat signal between the sent optical signal and the optical signal from a local oscillation semiconductor laser 2, a frequency discrimination circuit 30 with the S-shaped characteristic applies frequency discrimination to the beat signal, extracts a carrier signal component to control the frequency of the light of the local oscillation semiconductor laser 2 so as to maximize the amplitude of the carrier signal. Thus, the frequency of the light of the local oscillation semiconductor laser 2 traces with the frequency of the optical signal from the transmitter 100 and the original signal is stably obtained.