PURPOSE: To prevent increase in distortion at a relay point by eliminating a harmonic component of a sub signal to be sent, adding the result to a main signal subject to FM modulation, sending the added signal by the pulsized FM modulation system, and extracting the sub signal as a signal component of a frequency band of a sub signal component through filtering a pulsed FM signal.

CONSTITUTION: A main signal to be sent is FM modulated and a signal component of a same frequency band as that of a sub signal component is eliminated through an HPF 33, and the sub signal to be sent is given to an LPF 34 to eliminate a signal component of the same frequency band as that of the main signal component. The main and the sub signals through the HPF 33 and the LPF 34 are added and synthesized, the synthesis signal is sliced at a mean level to obtain a pulsed FM signal and a light source is driven by the pulsed FM signal to execute optical transmission. Thus, the increase in distortion due to an FM modulator and an FM demodulator at a relay point is evaded and the circuit constitution is simplified by the elimination of the FM modulator and the FM demodulator.