To solve the problem that each device connected after an optoeletric conversion part needs to have wide band characteristics corresponding to the whole frequency-multiplexed signal when digital modulated signals are optically transmitted by frequency-division multiplexing.

The light outputted from a light source 130 is transmitted after SSB modulation using the signal generated by applying ASK modulation to pieces 11 to 1n of digital data by respective ASK modulation parts 111 to 11n and then multiplexing the modulate signals. An optical filter part 350 separates a light carrier component and a light side-band component from the transmitted light signal. The light side-band component is multiplexed again with the light carrier component after SSB modulation by using a local oscillation signal having a frequency equal to the carrier frequency corresponding to the desired digital data. The multiplexed light signal is subjected to the squaring detection of an optoelectric conversion part 170 to obtain desired modulated digital data.